usb: gadget: Use unaligned access for wMaxPacketSize

Use get_unaligned() while fetching wMaxPacketSize to avoid
voilating any alignment rules.

Signed-off-by: Vivek Gautam <gautam.vivek@samsung.com>
Cc: Lukasz Majewski <l.majewski@samsung.com>
Cc: Piotr Wilczek <p.wilczek@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Lukasz Dalek <luk0104@gmail.com>
Cc: Marek Vasut <marex@denx.de>