USB: serial: replace runtime overflow check

Since commit 0a8fd134 ("USB: fix problems with duplicate endpoint
addresses") USB core guarantees that there are no more than 15 endpoint
descriptors per type (and altsetting) so the corresponding overflow
checks can now be replaced with a compile-time check on the array sizes
(and indirectly the maximum number of ports).

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