USB: serial: mos7840: clean up endpoint sanity check

Clean up the endpoint sanity check by letting core verify the single
interrupt endpoint, and verifying the bulk endpoints in calc_num_ports
after having determined the number of ports.

Note that the static type num_ports field was neither correct or used
(since calc_num_ports never returns zero).

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