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 <[e-mail]>
Signed-off-by: Richard Alpe <[e-mail]>
Tested-by: Jeff Pieper <[e-mail]>
Signed-off-by: Jeff Kirsher <[e-mail]>

drivers/net/ethernet/intel/e1000e/82571.c

···
  **/
 static s32 e1000_reset_hw_82571(struct e1000_hw *hw)
 {
-	u32 ctrl, ctrl_ext;
+	u32 ctrl, ctrl_ext, eecd;
 	s32 ret_val;
 
 	/*
···
 	 */
 
 	switch (hw->mac.type) {
+	case e1000_82571:
+	case e1000_82572:
+		/*
+		 * REQ and GNT bits need to be cleared when using AUTO_RD
+		 * to access the EEPROM.
+		 */
+		eecd = er32(EECD);
+		eecd &= ~(E1000_EECD_REQ | E1000_EECD_GNT);
+		ew32(EECD, eecd);
+		break;
 	case e1000_82573:
 	case e1000_82574:
 	case e1000_82583: