rsi: fix static checker warning

u32 pointer is changed to u16 and filled the value. Problem is
solved by using local temporary variable. Below static checker
warning was reported.

drivers/net/wireless/rsi/rsi_91x_usb.c:400 rsi_usb_master_reg_read()
        warn: passing casted pointer 'value' to 'rsi_usb_reg_read()'
32 vs 16.

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>