net: macb driver, check for SKBTX_HW_TSTAMP

Make sure SKBTX_HW_TSTAMP (i.e. SOF_TIMESTAMPING_TX_HARDWARE) has been
enabled for this skb. It does fix the issue where normal socks that
aren't expecting a timestamp will not wake up on select, but when a
user does want a SOF_TIMESTAMPING_TX_HARDWARE it does work.

Signed-off-by: Paul Thomas <pthomas8589@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>