staging:rtl8192u: Remove member diff_TH - Style

The member variable diff_TH is assigned a constant value and then used
in a comparison. The variable is never changed so the comparison can
as easily be performed directly with the defined constant.

The member variable has been removed and the defined constant
RxPathSelection_diff_TH renamed to RX_PATH_SELECTION_DIFF_TH, to clear
the checkpatch issue with CamelCase naming.

These are coding style changes which should have no impact on runtime
code execution.

Signed-off-by: John Whitmore <johnfwhitmore@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>