rt2x00: rt2800lib: don't hardcode beacon offsets

The values written into the BCN_OFFSET[01] registers are
hardcoded in the rt2800_init_register function.

Add a macro and a helper function to derive these values
directly from the base address of a given beacon, and use
the new function instead of the hardcoded numbers.

The patch contains no functional changes. The programmed
register values are the same before and after the patch.

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>