rtl8xxxu: gen1: Set aggregation timeout (REG_RXDMA_AGG_PG_TH + 1) as well

gen2 chips as well as 8188eu seems to use this register for setting
DMA timeout threshold values, however the 8192cu is using
REG_USB_DMA_AGG_TO. Set both to be on the safe side.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>