staging:rtl8192u: Rename ACI_AIFSN - Style

Rename the union ACI_AIFSN to aci_aifsn and remove the typedef directive.

The removal of the typedef clears the checkpatch issue with defining
new types. The renaming is to adhere to the coding style where types
are name in lower case.

These changes are coding style changes which should have no impact on
runtime execution.

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