staging: rtl8723au: Make direct calls for HW_VAR_TXDMA_AGG_PG_TH

This was in fact turned into a no-op for the RTL8723AU, but it looks
like a bug, so this reenables it.

Keep an eye out for side-effects here and revert this, if it causes
issues.

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