Input: maple_keyb - use guard notation when acquiring mutex

Using guard notation makes the code more compact and error handling
more robust by ensuring that mutexes are released in all code paths
when control leaves critical section.

Link: https://lore.kernel.org/r/Zx8mGiWOw1Av28TX@google.com


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>