gpio/pca953x: Fix IRQ support.

It seems that in the normal case, IRQ_NOREQUEST needs to be explicitly
cleared, otherwise claiming the interrupt fails.
In the case of sparse interrupts, the descriptor needs to be allocated
first.

Signed-off-by: David Jander <david@protonic.nl>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>