staging: r8188eu: use standard error codes in rtw_read_port

Replace the driver-specific _SUCCESS and _FAIL error codes in
rtw_read_port. Return 0 for success or a negative error code as
we do elsewhere in the kernel.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com> # Edimax N150
Link: https://lore.kernel.org/r/20230206201800.139195-3-martin@kaiser.cx


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>