usb: gadget: storage_common: make attribute operations more generic

Show/store methods for sysfs attributes contain code which can be used
also by configfs. Make them abstract the source the lun and rw_semaphore
are taken from.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>