leds: leds-pm8xxx: use snprintf for sysfs attribute show method

Sysfs attribute show method is passed a buffer of length PAGE_SIZE.
To prevent from overwriting this buffer, replace usage of sprintf()
with snprintf().

Change-Id: I0f12a2e1309940d0971dab1e7a29c9c03424a682
Signed-off-by: Himanshu Aggarwal <haggarwa@codeaurora.org>