rt2x00: Fix beacon skew in rt2800pci

rt2800pci is suffering from beacon skew in AP mode. Some powersaving
clients (like VOIP phones) are getting into trouble after some time
when the beacon skew is getting too big.

The ralink legacy drivers contain a function that indicates that the
beacon timer is off by 1us per tbtt. And this function works around
that by reducing the beacon interval for every 64th beacon transmission
by 64us (the smallest possible value). Do the same in rt2800pci.

This allows proper powersaving when rt2800pci is used in AP mode.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>