usb: dwc3-msm: Simplify GSI register macros

The GSI register macros require looking up the offset from the
gsi_reg array first. We can simplify the usages by including the
index lookup within the macro itself, which saves some repetition.

Change-Id: Ie7c2b2b09799f3afaefd598626884afcfc769e86
Signed-off-by: Jack Pham <jackp@codeaurora.org>
Signed-off-by: Pratham Pratap <prathampratap@codeaurora.org>