wimax: i2400m - prevent a possible kernel bug due to missing fw_name string

This happened on a machine with a custom hotplug script calling nameif,
probably due to slow firmware loading. At the time nameif uses ethtool
to gather interface information, i2400m->fw_name is zero and so a null
pointer dereference occurs from within i2400m_get_drvinfo().

Signed-off-by: Phil Sutter <phil.sutter@viprinet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>