rtl8xxxu: gen1: Add module parameters to adjust DMA aggregation parameters

This allows the user to specify DMA aggregation timout and block
count. Blocks are presumably always 512 bytes, so the minimum block
count is 6 for 802.11 packets.

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