usb: gadget: f_mass_storage: make sysfs interface optional

usb: gadget: f_mass_storage: make sysfs interface optional

When configfs is in place, the luns will not be represented in sysfs,
so there will be no struct device associated with a lun.
In order to maintain compatibility and allow configfs adoption
sysfs is made optional in this patch.

As a consequence some debug macros need to be adjusted. Two new
fields are added to struct fsg_lun: name and name_pfx.
The "name" is for storing a string which is presented to the user
instead of the dev_name. The "name_pfx", if non-NULL, is prepended
to the "name" at printing time.

The name_pfx is for a future lun.0, which will be a default group in
mass_storage.<name>. By design at USB function configfs group's creation
time its name is not known (but instead set a bit later in
drivers/usb/gadget/configfs.c:function_make) and it is this name that
serves the purpose of the said name prefix. So instead of copying
a yet-unknown string a pointer to it is stored in struct fsg_lun.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>