xilinx_axienet: Fix bit mask code

Obviously the code wanted to mask the lower bits but failed to do so
because of a missing "<".

cppcheck detected a conditional expression which was always true (1 < 7).
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NEdgar E. Iglesias <edgar.iglesias@gmail.com>

hw/net/xilinx_axienet.c

@@ -575,7 +575,7 @@ static void enet_write(void *opaque, hwaddr addr,
             break;
 
         case R_MC:
-             value &= ((1 < 7) - 1);
+             value &= ((1 << 7) - 1);
 
              /* Enable the MII.  */
              if (value & MC_EN) {