greybus: Remove "gb-" prefix from .c files

Some files are still prefixed with "gb-" with the reasoning that the modules
would be named so, i.e.  gb-*.ko. But this can be done by playing a bit in
Makefile instead and keep uniform naming of .c files.

Lets try it.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>