ITE drivers/timer: clean up sys_clock_set_timeout()

Setting event timer count at least 1 hw count, it's redundant,
so I clean up this else {} case. And add the comment about
the K_TICKS_FOREVER and INT_MAX case.

NOTE:
CONFIG_TIMEOUT_64BIT = y, then k_ticks_t type is int64_t.
K_FOREVER is (k_timeout_t) { .ticks = (K_TICKS_FOREVER) },
and K_TICKS_FOREVER is ((k_ticks_t) -1),
so K_FOREVER is a k_timeout_t type structure, and
the member ticks: type int64_t,
                  value (= K_TICKS_FOREVER) 0xFFFF FFFF FFFF FFFF.

Signed-off-by: Ruibin Chang <Ruibin.Chang@ite.com.tw>