Input: adp5589-keys - fix event count mask

The event mask was specified as 0xF (4 bits) when in reality is 0x1F (5
bits) in order to be capable of representing all FIFO length values from
0 to 16.

This caused a problem: when the keypad reported 16 pending events the
driver took it as 0, and did nothing. This in turn caused the keypad to
re-issue the interrupt over and over again.

Signed-off-by: Guido Martínez <guido@vanguardiasur.com.ar>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>