staging:rtl8192u: Rename variable MaxTxPowerInDbm - Style

Rename the variable MaxTxPowerInDbm to max_tx_pwr_dbm. This change clears a
checkpatch issue with CamelCase naming. This coding style change should not
impact runtime execution.

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