usb: dwc3: gadget: remove redundant variable maxpacket

Variable maxpacket is being assigned but is never used hence it is
redundant and can be removed.

Cleans up clang warning:
warning: variable 'maxpacket' set but not used [-Wunused-but-set-variable]

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>