rt2800: use BBP_R1 for setting tx power

TX power delta can be negative. TX_PWR_CFG_ registers allow to set delta
only in range between 0 dBm and 15 dBm (4 bits for each rate). Se we
need to use BBP_R1 to configure negative deltas.

Not utilize +6 dBm increasing BBP_R1 option for safety reason. For now,
this can be used for devices, which export maximum allowed TX power
value.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Helmut Schaa <helmut.schaa@googlemail.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>