staging/rtl8187se: Remove duplicated code by using an offset

In SetAntennaConfig87SE both branches of if (bAntDiversity) do exactly
the same, except that there is an offset of 0x80 for the register values
if bAntDiversity is true.

-> Consolidate both branches and assign the offset if necessary.

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>