drivers: mcux_os_timer: Refactor ISR

Refactor ISR by:
1) making a helper function to put the tickful calculation in, to reduce
   an indentation level and cognitively separate this case while reading
2) Rewrite the calculations to simplify them. More info on that below.

Math changes:
- In the ISR, there was a variable called "now" which represented the
current cycle count of the timer. last_count was updated using a
calculation based on the number of elapsed ticks
and the hardware cycles per second value. However, this is
redundant because mathematically, this is equivalent to "now". So the
variable can just be set to now.
Proof is that the previous calculation was (in programming language):
(1) last_count = last_count + elapsed_ticks * CYC_PER_TICK
So let's rewrite this as the following to be clear about the different
values we are talking about here (in math language):
(2) count[t] = count[t-1] + elapsed_ticks * CYC_PER_TICK
We calculated elapsed ticks in the function as:
(3) elapsed_ticks = (now - last_count) / CYC_PER_TICK
(4) elapsed_ticks = (now - count[t-1]) / CYC_PER_TICK
Rearranging (2), we see
(5) elapsed_ticks = ( count[t] - count[t-1] ) / CYC_PER_TICK
Substituting this into (4), we get:
(6) ( count[t] - count[t-1] ) / CYC_PER_TICK =
			(now - count[t-1]) / CYC_PER_TICK
Doing simple algebra, you can see we result with:
(7) count[t] = now
So therefore, we can simplify the programming expression to
(8) last_count = now

- The other change is regarding the calculation of the next tick match
  value for tickful kernel mode. The previous calculation was doing:
(1) next = now + CYC_PER_TICK
We know that last_count is equivalent to now, at this point in the code
with the first change:
(2) next = last_count + CYC_PER_TICK
And then, for some reason, we are adding yet another CYC_PER_TICK to
next if (next - now) < MIN_DELAY. The reason for that I do not
understand, but let's write that down:
(3) next - now < MIN_DELAY
Now rewrite (2) as (4) and (3) as (5):
(4) count[t+1] = count[t] + CYC_PER_TICK
(5) count[t+1] - count[t] < MIN_DELAY
And now we substitute (4) into (5):
(6) count[t] + CYC_PER_TICK - count[t] < MIN_DELAY
And simplify:
(7) CYC_PER_TICK < MIN_DELAY
So actually no runtime calculations are needed here, this is a
hardcoding. The reason for this calculation I don't know, but we can
simplify the code.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>