rsi: buffer available interrupt handling

BUFFER_AVAILABLE interrupt is sent by firmware to indicate
change in buffer status. We should check buffer status while
handling this interrupt. Currently buffer status is checked
only while dequeueing packets. This patch fixes a data
traffic stuck problem observed occasionally.

Signed-off-by: Karun Eagalapati <karun256@gmail.com>
Signed-off-by: Amitkumar Karwar <amit.karwar@redpinesignals.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>