staging:rtl8192u: Rename structure member FirstChnl - Style

Rename structure member FirstChnl to first_channel. This coding style change
clears a checkpatch issue with CamelCase naming. This change should not impact
the runtime code execution.

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