leds: cht-wcove: Fix an unsigned comparison which can never be negative

The return value from the call to cht_wc_leds_find_freq() is int.
However, the return value is being assigned to an unsigned
int variable 'ctrl', so making it an int.

Eliminate the following warning:
drivers/leds/leds-cht-wcove.c:236 cht_wc_leds_set_effect() warn: unsigned 'ctrl' is never less than zero.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Closes: https://bugzilla.openanolis.cn/show_bug.cgi?id=5341


Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230531020238.102684-1-yang.lee@linux.alibaba.com