timer: ambiq_stimer: Correct set_timeout's delta clock calculation

In sys_clock_set_timeout(), input "ticks" is used to compute next
timeout point, Ambiq's STimer API used to sets next timeout has input
parameter as ui32Delta, which inside the API is using
"this value to add to the STimer counter and load into the comparator
register" according to its spec, thus the this delta clock is
almost equivalent to input "ticks"'s concept, and is not related to
last_count, it should be computed directly from input "ticks".
This correction fixes the test case failure at
zephyr\tests\kernel\tickless\tickless_concept.

Signed-off-by: Bryan Zhu <bzhu@ambiq.com>