rsi: use separate mutex lock for receive thread

Deadlock issue is observed during our stress tests. The root
cause for the issue is same lock is used between tx and rx threads.

This patch adds a separate mutex lock for rx thread to resolve
the problem.

Signed-off-by: Karun Eagalapati <karun256@gmail.com>
Signed-off-by: Amitkumar Karwar <amit.karwar@redpinesignals.com>
Signed-off-by: Prameela Rani Garnepudi <prameela.j04cs@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>