rsi: use subdirectory for firmware file

linux-firmware maintainers prefer to have a subdirectory for
firmware files. Code is changed here to pick firmware file from
a subdirectory. Firmware file with a new loading mechanism will
be submitted inside rsi directory.

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