USB: serial: keyspan_pda: verify endpoints at probe

Check for the expected endpoints in attach() and fail loudly if not
present.

Note that failing to do this appears to be benign since da280e34
("USB: keyspan_pda: clean up write-urb busy handling") which prevents a
NULL-pointer dereference in write() by never marking a non-existent
write-urb as free.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Cc: stable <stable@vger.kernel.org>	# < v3.3
Signed-off-by: Johan Hovold <johan@kernel.org>