staging:rtl8192u: Rename ToLegalChannel - Style

Rename the function ToLegalChannel, which causes a checkpatch issue due
to its use of CamelCase naming. The function has been renamed to
to_legal_channel.

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>