staging: r8188eu: limit rtw_dbg_port to path a

Rf registers can be read by rtw_wx_read_rf or via the private ioctl 0x0B.
The latter calls rtw_dbg_port. Limit rf register reads to path a in
rtw_debug_port.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20220212161737.381841-8-martin@kaiser.cx


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