staging: rtl8723au: Remove WMM_ defines

Only one is used, and only describes the size of an array defined in a
local function. Use sizeof() in the local function to deobfuscate it.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>