net: w5100: increase TX timeout period

This increases TX timeout period from one second to 5 seconds which is
the default value if the driver doesn't explicitly set
net_device->watchdog_timeo.

The one second timeout is too short for W5100 with SPI interface mode
which doesn't support burst READ/WRITE processing in the SPI transfer.
If the packet is transmitted while RX packets are being received at a
very high rate, the TX transmittion work in the workqueue is delayed
and the watchdog timer is expired.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Mike Sinkovsky <msink@permonline.ru>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: David S. Miller <davem@davemloft.net>