usb: gadget: Add per-lun inquiry string

Introduce an attribute "inquiry_string" to the lun.

In some environments, e. g. BIOS boot menus, the inquiry string
is the only information about devices presented to the user. The
default string depends on the "cdrom" bit of the first lun as
well as the kernel version and allows no further customization.
So without access to the client it is not obvious which gadget is
active at a given point and what any of the available luns might
contain.

If "inquiry_string" is ignored or set to the empty string, the
old behavior is preserved.

Signed-off-by: Philipp Gesang <philipp.gesang@intra2net.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>