staging: r8188eu: add error handling of rtw_read8

staging: r8188eu: add error handling of rtw_read8

rtw_read8() reads data from device via USB API which may fail. In case
of any failure previous code returned stack data to callers, which is
wrong.

Fix it by changing rtw_read8() prototype and prevent caller from
touching random stack data

Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Link: https://lore.kernel.org/r/c8f8ef4f14db3ba2478a87d5be6eb768a093dfaf.1654629778.git.paskripkin@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>