leds: lp5521/5523: Remove duplicate mutex

It can be a problem when a pattern is loaded via the firmware interface.
LP55xx common driver has already locked the mutex in 'lp55xx_firmware_loaded()'.
So it should be deleted.

On the other hand, locks are required in store_engine_load()
on updating program memory.

Reported-by: Pali Rohár <pali.rohar@gmail.com>
Reported-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Milo Kim <milo.kim@ti.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>
Cc: <stable@vger.kernel.org>