usb: gadget: loopback: fix: Don't share qlen and buflen between instances

Each instance of loopback function may have different qlen
and buflen attributes values. When linking function to
configuration those values had been assigned to global
variables. Linking other instance to config overwrites those
values.

This commit moves those values to f_loopback structure
to avoid overwriting. Now each function has its own instance
of those values.

Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Reviewed-by: Robert Baldyga <r.baldyga@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>