greybus: firmware: prefix char-device with 'gb-'

This will make it consistent with any other character devices we have
for greybus and let us identify greybus character devices easily.

Compiled tested only.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>