e1000e: test for valid check_reset_block function pointer

commit 44abd5c12767a8c567dc4e45fd9aec3b13ca85e0 introduced NULL pointer
dereferences when attempting to access the check_reset_block function
pointer on 8257x and 80003es2lan non-copper devices.

This fix should be applied back through 3.4.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
diff --git a/drivers/net/ethernet/intel/e1000e/phy.c b/drivers/net/ethernet/intel/e1000e/phy.c
index 0334d01..b860d4f 100644
--- a/drivers/net/ethernet/intel/e1000e/phy.c
+++ b/drivers/net/ethernet/intel/e1000e/phy.c
@@ -2155,9 +2155,11 @@
 	s32 ret_val;
 	u32 ctrl;
 
-	ret_val = phy->ops.check_reset_block(hw);
-	if (ret_val)
-		return 0;
+	if (phy->ops.check_reset_block) {
+		ret_val = phy->ops.check_reset_block(hw);
+		if (ret_val)
+			return 0;
+	}
 
 	ret_val = phy->ops.acquire(hw);
 	if (ret_val)