mcux_os_timer: Refactor elapsed ticks calculation

There are a couple places where effectively the exact same calculation
is done. Refactor to share code to do this calculation to make it easier
to read, and explode the code to multiple statements to be even easier
to follow the calculation. Compiler will likely optimize the same way
when opt enabled. Rename dticks variable to be more explicit what that
means (elapsed ticks).

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