rt2x00: Update comment on freq_offset field in struct rt2x00_dev.

rt2x00: Update comment on freq_offset field in struct rt2x00_dev.

The comment states that the field is only used for rt61pci and rt73usb.
However, it is now used by rt2800pci and rt2800usb as well, so the
comment is not correct anymore.

Update the comment to not state any low-level drivers anymore.

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