rsi_91x: fix uninitialized variable

There is a potential execution path in which variable ret is returned
without being properly initialized previously.

Fix this by storing the value returned by function
rsi_usb_master_reg_write into _ret_.

Addresses-Coverity-ID: 1468407 ("Uninitialized scalar variable")
Fixes: 16d3bb7b ("rsi: disable fw watchdog timer during reset")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>