ar71xx: ag71xx: make switch register access atomic

ar71xx: ag71xx: make switch register access atomic

Reading of the PHY registers occasionally returns with bogus values
under heavy load. This misleads the PHY driver and thus causes false
link/speed change notifications which leads to performance loss.

This is easily noticable during an iperf session:

...
[  3] 52.0-53.0 sec  11.3 MBytes  94.4 Mbits/sec
[  3] 53.0-54.0 sec  11.4 MBytes  95.4 Mbits/sec
eth1: link down
br-lan: port 2(eth1) entering forwarding state
eth1: link up (100Mbps/Full duplex)
br-lan: port 2(eth1) entering forwarding state
br-lan: port 2(eth1) entering forwarding state
[  3] 54.0-55.0 sec  6.75 MBytes  56.6 Mbits/sec
[  3] 55.0-56.0 sec  0.00 Bytes  0.00 bits/sec
[  3] 56.0-57.0 sec  10.5 MBytes  88.1 Mbits/sec
...
[  3] 169.0-170.0 sec  11.4 MBytes  95.4 Mbits/sec
[  3] 170.0-171.0 sec  11.4 MBytes  95.4 Mbits/sec
eth1: link up (10Mbps/Half duplex)
[  3] 171.0-172.0 sec  7.63 MBytes  64.0 Mbits/sec
[  3] 172.0-173.0 sec  9.38 MBytes  78.6 Mbits/sec
eth1: link up (100Mbps/Full duplex)
[  3] 173.0-174.0 sec  11.3 MBytes  94.4 Mbits/sec
[  3] 174.0-175.0 sec  11.4 MBytes  95.4 Mbits/sec

SVN-Revision: 26856