atl1c: fix issue of io access mode for AR8152 v2.1

When io access mode is enabled by BOOTROM or BIOS for AR8152 v2.1,
the register can't be read/write by memory access mode.
Clearing Bit 8  of Register 0x21c could fixed the issue.

Signed-off-by: Cloud Ren <cjren@qca.qualcomm.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: xiong <xiong@qca.qualcomm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>