e1000e: clear REQ and GNT in EECD (82571 && 82572)

Clear the REQ and GNT bit in the eeprom control register (EECD).
This is required if the eeprom is to be accessed with auto read
EERD register.

After a cold reset this doesn't matter but if PBIST MAC test was
executed before booting, the register was left in a dirty state
(the 2 bits where set), which caused the read operation to time out
and returning 0.

Reference (page 312):
http://download.intel.com/design/network/manuals/316080.pdf

Reported-by: Aleksandar Igic <aleksandar.igic@dektech.com.au>
Signed-off-by: Richard Alpe <richard.alpe@ericsson.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>