net: fix endian check warning in etherdevice.h

Sparse builds have been warning for a really long time now
that etherdevice.h has a conversion that is unsafe.

  include/linux/etherdevice.h:79:32: warning: restricted __be16 degrades to integer

This code change fixes the issue and generates the exact
same assembly before/after (checked on x86_64)

Fixes: 2c722fe1 (etherdevice: Optimize a few is_<foo>_ether_addr functions)
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
CC: Joe Perches <joe@perches.com>
Signed-off-by: David S. Miller <davem@davemloft.net>