include: zephyr: sys: time_units: Fix precision bug in z_tmcvt_32

Fixing the precedence of the cast in z_tmcvt_int_div_32 and
z_tmcvt_int_mul_32 functions so the final result is of type
uint32_t.

Fixes #87226

Signed-off-by: Anisetti Avinash Krishna <anisetti.avinash.krishna@intel.com>