rtl818x: add comments to explain few not obvious HW configs.

Certain HW options (TX packet retry count, CW configuration and
TX power configuration) can be specified in both the TX packet
descriptor and also into HW "global" registers.

The HW is thus configured to honour the global register or the
TX descriptor field depending by the case.

This patch adds few comments that hopefully clarify in which cases
the driver uses one method and in which cases it uses the other.

Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>