rsi: Remove old firmware loading method

The older firmware loading method is not usable by any Redpine chipset.
Hence removing that part of the code. Older firmware image with
rsi_91x.fw name is deprecated

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