cns3xxx: fix GPIO controller interrupt enable

The cns3xxx interrupt controller uses a single register and as such
the 'mask' reg/functions must be used as opposed to the 'enable'/'disable'
reg/functions.

This fixes an issue that occurs if more than one GPIO on a specific controller
(there is GPIOA and GPIOB each having 32 GPIO's) uses interrupts. When one
would get enabled all others would be disabled prior to this patch.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Acked-by: Koen Vandeputte <koen.vandeputte@ncentric.com>