Input: mt6779-keypad - fix signedness bug

The irq variable is defined as unsigned int. If the platform_get_irq()
returns a negative value, data type cast may result in error.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Lv Ruyi (CGEL ZTE) <lv.ruyi@zte.com.cn>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Link: https://lore.kernel.org/r/20220308025054.2077162-1-lv.ruyi@zte.com.cn


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