Input: synaptics-rmi4 - remove check of Non-NULL array

Originally the irq_mask member of rmi_function was a pointer. Then it was
switched to being a zero length array. However, the checks for a NULL
pointer where not removed.

Signed-off-by: Andrew Duggan <aduggan@synaptics.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>