staging:rtl8192u: Remove unused enumerated type ACK_POLICY - Style

The enumerated type ACK_POLICY is not used in code so it has been removed
from the source code. This is a coding style change and 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>