target/usb-gadget: remove duplicate initialization

We set bAlternateSetting to zero twice.  I kept the second one
(.bAlternateSetting = USB_G_ALT_INT_BBB) because that's more
descriptive.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>