net: stmmac: modify default value of tx-frames

the default value of tx-frames is 25, it's too late when
passing tstamp to stack, then the ptp4l will fail:

ptp4l -i eth0 -f gPTP.cfg -m
ptp4l: selected /dev/ptp0 as PTP clock
ptp4l: port 1: INITIALIZING to LISTENING on INITIALIZE
ptp4l: port 0: INITIALIZING to LISTENING on INITIALIZE
ptp4l: port 1: link up
ptp4l: timed out while polling for tx timestamp
ptp4l: increasing tx_timestamp_timeout may correct this issue,
       but it is likely caused by a driver bug
ptp4l: port 1: send peer delay response failed
ptp4l: port 1: LISTENING to FAULTY on FAULT_DETECTED (FT_UNSPECIFIED)

ptp4l tests pass when changing the tx-frames from 25 to 1 with
ethtool -C option.
It should be fine to set tx-frames default value to 1, so ptp4l will pass
by default.

Signed-off-by: Biao Huang <biao.huang@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>