staging:rtl8192u: Remove unused enumerated type ACM_METHOD - Style

Remove the enumerated type ACM_METHOD as it is unused in code. This is
a coding style change and should not impact runtime code execution.

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