can: gs_usb: gs_make_candev(): fix memory leak for devices with extended bit timing configuration

Some CAN-FD capable devices offer extended bit timing information for
the data bit timing. The information must be read with an USB control
message. The memory for this message is allocated but not free()ed (in
the non error case). This patch adds the missing free.

Fixes: 6679f4c5

 ("can: gs_usb: add extended bt_const feature")
Link: https://lore.kernel.org/all/20220329193450.659726-1-mkl@pengutronix.de
Reported-by:  <syzbot+4d0ae90a195b269f102d@syzkaller.appspotmail.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>