USB: gadget: g_mass_storage: static data instead of dynamic allocation

This patch changes msg_do_config() function so that it uses
a static object for a fsg_common structure instead of dynamically
allocated.  This is a micro-optimisation.

Signed-off-by: Michal Nazarewicz <m.nazarewicz@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>