staging:rtl8192u: Rename IsLegalChannel - Style

The function IsLegalChannel causes a checkpatch issue due to its use
of CamelCase naming. The function has been renamed to is_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>