Input: pcf8574_keypad - fix off by one in pcf8574_kp_irq_handler()

If nextstate == ARRAY_SIZE(lp->btncode), then we read one past the end of
the array on the next line.

This fixes a smatch warning:
drivers/input/misc/pcf8574_keypad.c +74 pcf8574_kp_irq_handler(8)
	error: buffer overflow 'lp->btncode' 17 <= 17

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>