USB: gadget: f_mass_storage: use static attribute groups for sysfs entries

Instead of manual device_create_file() and device_remove_file() calls,
assign the static attribute groups to the lun device to register.
The RO or RW permissions for some entries are decided in is_visible
callback.  This simplifies the code (also the logic) and avoids the
possible races, too.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>