e1000: fix concurrent accesses to PHY from watchdog and ethtool

The e1000 driver currently does not protect concurrent accesses to the PHY
from both the ethtool callbacks, and from the e1000_watchdog function. This
patchs adds a new spinlock which is used by e1000_{read,write}_phy_reg in
order to serialize concurrent accesses to the PHY.

Signed-off-by: Maxime Bizon <mbizon@freebox.fr>
Signed-off-by: Florian Fainelli <ffainelli@freebox.fr>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>