net: faraday: ftmac100: Use BUG_ON instead of if condition followed by BUG.

Notice that in this particular case unlikely() is already being called
inside BUG_ON macro.

This issue was detected with the help of Coccinelle.

Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>