rt2x00: rt2800lib: add rt2800_hw_beacon_base helper

The HW_BEACON_BASE() macro returns the base address
of a given beacon, however the returned values are
not usable on all chipsets. On devices which have
selectable shared memory parts, some beacon may be
located in the high part of the shared memory.

Instead of extending the already complicated macro,
add a new helper function and use that to get the
base address of a given beacon.

The actual patch contains no functional changes, the
helper function will be extended in a further patch
to handle different chipsets' requirements.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Acked-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>