firmware: gsmi: remove sysfs entries when unload the module

This patch removes sysfs entries in gsmi_exit() and gsmi_init() error path.

Also move the driver successfully loaded message to the end of gsmi_init()
and return proper error if register_efivars() fails.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>