wimax/i2400m: fix race condition with tcpdump et al

tcpdump and friends were not being able to decode packets sent via
WiMAX; they had a zero ethernet type, even when the stack was properly
sending them to the device with the right type.

It happens that the driver was overwriting the (fake) ethernet header
for creating the hardware header and that was bitting the cloning used
by tcpdump (et al) to look into the packets.

Use pkskb_expand_head() [method copied from the e1000 driver] to fix.

Thanks to Herbert Xu and Andi Kleen for helping to diagnose and
pointing to the right fix.

Cc: Herbert Xu <gondor.apana.org.au>
Cc: Andi Kleen <andi@firstfloor.org>
Signed-off-by: Inaky Perez-Gonzalez <inaky@linux.intel.com>