drivers: timer: adsp: Improve elapsed ticks calculations

It is better to use 64-bit variable types for calculating the number
of elapsed ticks than 32-bit variable types. This guards against the
propagation of calculation errors should the lower 32-bits of the timer
counter roll over multiple times before the timer ISR is serviced.

(Such a scenario can easily occur when pausing the system for an
extended period of time with a debugging device such as a Lauterbach.)

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>