Input: tca8418_keypad - hide gcc-4.9 -Wmaybe-uninitialized warning

Older versions of gcc warn about the tca8418_irq_handler function
as they can't keep track of the variable assignment inside of the
loop when using the -Wmaybe-unintialized flag:

drivers/input/keyboard/tca8418_keypad.c: In function ‘tca8418_irq_handler’:
drivers/input/keyboard/tca8418_keypad.c:172:9: error: ‘reg’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
drivers/input/keyboard/tca8418_keypad.c:165:5: note: ‘reg’ was declared here

This is fixed in gcc-6, but it's possible to rearrange the code
in a way that avoids the warning on older compilers as well.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>