net/smsc911x: Fix rare soft reset timeout issue due to PHY power-down mode

The patch affect SMSC LAN generation 4 chips with integrated PHY (LAN9221).

It is possible that PHY could enter power-down mode (ENERGYON clear),
between ENERGYON bit check in smsc911x_phy_disable_energy_detect and SRST
bit set in smsc911x_soft_reset. This could happen, for example, if someone
disconnect ethernet cable between the checks. The PHY in a power-down mode
would prevent the MAC portion of chip to be software reseted.

Initially found by code review, confirmed later using test case.

This is low probability issue, and in order to reproduce it you have to
run the script:

while true; do
	ifconfig eth0 down
	ifconfig eth0 up || break
done

While the script is running you have to plug/unplug ethernet cable many
times (using gpio controlled ethernet switch, for example) until get:

[ 4516.477783] ADDRCONF(NETDEV_UP): eth0: link is not ready
[ 4516.512207] smsc911x smsc911x.0: eth0: SMSC911x/921x identified at 0xce006000, IRQ: 336
[ 4516.524658] ADDRCONF(NETDEV_UP): eth0: link is not ready
[ 4516.559082] smsc911x smsc911x.0: eth0: SMSC911x/921x identified at 0xce006000, IRQ: 336
[ 4516.571990] ADDRCONF(NETDEV_UP): eth0: link is not ready
ifconfig: SIOCSIFFLAGS: Input/output error

The patch was reviewed by Steve Glendinning and Microchip Team.

Signed-off-by: Alexander Kochetkov <al.kochet@gmail.com>
Acked-by: Steve Glendinning <steve.glendinning@shawell.net>
Signed-off-by: David S. Miller <davem@davemloft.net>