rt2800: allow to reduce tx power on devices not exporting power limit

Some rt2800 devices don't have their calibrated max eirp tx power in
their calibration data. For those devices reduce tx power according to
difference between regulatory max channel power and requested tx power.

This patch is based on Helmut Schaa work.

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