net: wimax/i2400m: fix NULL-deref at probe

Make sure to check the number of endpoints to avoid dereferencing a
NULL-pointer or accessing memory beyond the endpoint array should a
malicious device lack the expected endpoints.

The endpoints are specifically dereferenced in the i2400m_bootrom_init
path during probe (e.g. in i2400mu_tx_bulk_out).

Fixes: f398e424 ("i2400m/USB: probe/disconnect, dev init/shutdown
and reset backends")
Cc: Inaky Perez-Gonzalez <inaky@linux.intel.com>

Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>