staging:rtl8192u: Rename constants RateAdaptiveTH_Low_* - Style

The two constants, RateAdaptiveTH_Low_20M and RateAdaptiveTH_Low_40M
generate a checkpatch warning about CamelCase naming. The two have been
renamed to clear this issue. RATE_ADAPTIVE_TH_LOW_20M and
RATE_ADAPTIVE_TH_LOW_40M

This is a coding style change 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>