USB: serial: ti_usb_3410_5052: drop bogus read urb check

The read urb pointer is dereferenced before checking that it is non-NULL
during open(), but no check is needed as the existence of a bulk in
endpoint is verified during attach() since commit ef079936 ("USB:
serial: ti_usb_3410_5052: fix NULL-deref at open").

Drop the bogus read urb sanity check.

Signed-off-by: Johan Hovold <johan@kernel.org>