cxgbe/iw_cxgbe: c4iw_zero_addr should not use IN_ZERONET.

c4iw_zero_addr is supposed to check for all-zero addresses but was using
IN_ZERONET (which does something different) for IPv4 addresses. Fix it
by simply checking for 0 as is done for IPv6 addresses.

Reported by: karels@
MFC after: 3 days
Sponsored by: Chelsio Communications