media: flexcop-usb: fix endpoint sanity check

commit 1b976fc6 ("media: b2c2-flexcop-usb: add sanity checking") added
an endpoint sanity check to address a NULL-pointer dereference on probe.
Unfortunately the check was done on the current altsetting which was later
changed.

Fix this by moving the sanity check to after the altsetting is changed.

Fixes: 1b976fc6 ("media: b2c2-flexcop-usb: add sanity checking")
Cc: Oliver Neukum <oneukum@suse.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>