USB: serial: io_edgeport: fix NULL-deref at open

Fix NULL-pointer dereference when initialising URBs at open should a
non-EPIC device lack a bulk-in or interrupt-in endpoint.

Unable to handle kernel NULL pointer dereference at virtual address 00000028
...
PC is at edge_open+0x24c/0x3e8 [io_edgeport]

Note that the EPIC-device probe path has the required sanity checks so
this makes those checks partially redundant.

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