staging:rtl8192u: Rename variable NumChnls - Style

Rename the member variable NumChnls to num_channels. This change clears the
checkpatch issue with CamelCase naming. The change should not impact runtime
execution.

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