leds: simple_strtoul() cleanup

led-class.c and ledtrig-timer.c still use simple_strtoul().  Change them
to use kstrtoul() instead of obsolete simple_strtoul().

Also fix the existing int ret declaration to be ssize_t to match the
return type for _store functions in ledtrig-timer.c.

Signed-off-by: Shuah Khan <shuahkhan@gmail.com>
Cc: Joe Perches <joe@perches.com>
Cc: Richard Purdie <rpurdie@rpsys.net>
Cc: Bryan Wu <bryan.wu@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>