usb: gadget: zero: Fix SuperSpeed enumeration for alternate setting 1

It was impossible to enumerate on a SuperSpeed (XHCI) host
with alternate setting = 1 due to the wrongly set 'bMaxBurst'
field in the SuperSpeed Endpoint Companion descriptor.

Testcase:
<host> modprobe -r usbtest; modprobe usbtest alt=1
<device> modprobe g_zero
plug device to SuperSpeed port on the host.

Without this patch the host always complains like so
"usb 12-2: Not enough bandwidth for new device state.
 usb 12-2: Not enough bandwidth for altsetting 1"

Bug was introduced by commit cf9a08ae in v3.9

Fixes: cf9a08ae (usb: gadget: convert source sink and loopback to
new function interface)

Cc: 3.9+ <stable@vger.kernel.org> # 3.9+
Reviewed-by: Felipe Balbi <balbi@ti.com>
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>