net: ipa: add a missing __iomem attribute

The virt local variable in gsi_channel_state() does not have an
__iomem attribute but should.  Fix this.

Signed-off-by: Alex Elder <elder@linaro.org>
Reviewed-by: Amy Parker <enbyamy@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>