rsi: remove redundant flash_content variable

while cleaning up the driver, observed that flash_content pointer is not
necessary in rsi_load_firmware(). Instead of this, driver can use
'fw_entry->data' directly.Hence, removed redundant flash_content pointer.

Signed-off-by: Siva Rebbagondla <siva.rebbagondla@redpinesignals.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>