usb:gadget:f_thor: Allocate request up to THOR_PACKET_SIZE not ep->maxpacket

Now it is possible to allocate static request - which receives data from
the host (OUT transaction) to the size of THOR packet.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Cc: Marek Vasut <marex@denx.de>