ar8327: Add workarounds for AR8337 switch.

RGMII RX delay setting needs to be always specified for AR8337 to
avoid port 5 RX hang on high traffic / flood conditions.

Also, the HOL registers that set per-port and per-packet-priority
buffer sizes are updated with the reduced values suggested by the
QCA switch team.

Finally, AR8327 reserved register fixups are disabled for the AR8337.

This patch is adapted from the Code Aurora QSDK, but with magic
values mapped to proper defines.

Signed-off-by: Vittorio Gambaletta <openwrt@vittgam.net>