regmap-irq: do not write mask register if mask_base is zero

commit 7151449fe7fa5962c6153355f9779d6be99e8e97 upstream.

If client have not provided the mask base register then do not
write into the mask register.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Jinyoung Park <jinyoungp@nvidia.com>
Signed-off-by: Venkat Reddy Talla <vreddytalla@nvidia.com>
Signed-off-by: Mark Zhang <markz@nvidia.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>