rsi: buffer full check optimization

We get buffer full event from firmware whenever Tx queue is full
Host should stop writing packets after this and resume after
buffer free event.
Buffer status checking is optimized for once in 4 times if
BUFF_FULL condition is not set, otherwise once for every
packet.

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>