net: ipa: share field mask values for GSI interrupt type

The GSI interrupt type register and interrupt type mask register
have the same field bits at the same locations.  Use a common set of
field masks for both registers rather than essentially duplicating
them.  The only place the interrupt mask register uses any of these
is in gsi_irq_enable().

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>