bus: brcmstb_gisb: Use register offsets with writes too

This commit corrects the bug introduced in commit f8083587
("bus: brcmstb_gisb: Look up register offsets in a table") such
that gisb_write() translates the register enumeration into an
offset from the base address for writes as well as reads.

Fixes: f8083587 ("bus: brcmstb_gisb: Look up register offsets in a table")
Signed-off-by: Doug Berger <opendmb@gmail.com>
Acked-by: Gregory Fong <gregory.0xf0@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>