rsi: increase max supported aggregation subframes

Maximum number of supported aggregation subframes has been increased
to 8. This is the optimal number for the driver.

Signed-off-by: Amitkumar Karwar <amit.karwar@redpinesignals.com>
Signed-off-by: Siva Rebbagondla <siva.rebbagondla@redpinesignals.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>