rsi: check length before USB read/write register

These checks are required. Otherwise we may end up getting
memory corruption if invalid length is passed.

Fixes: b97e9b94 ("rsi: Add new host interface operations")
Signed-off-by: Amitkumar Karwar <amit.karwar@redpinesignals.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>