staging: rtl8723bs: fix u8 less than zero check

The error variable ret is currently a u8 and so two comparisons
to see if an error return is less than zero will always be false
because ret is unsigned.  Fix this by making ret an int.

Fixes: 554c0a3a ("staging: Add rtl8723bs sdio wifi driver")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>