staging:rtl8192u: Rename Channel - Style

The function dot11d_get_max_tx_pwr_in_dbm() uses a parameter name,
Channel, which causes a checkpatch issue with CamelCase naming. The
parameter has been renamed to channel.

The change 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>