staging: r8188eu: use a define for the radio path

Replace (enum rf_radio_path)0 with RF_PATH_A. This should make it
easier to see which path is selected.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20220205113741.379070-8-martin@kaiser.cx


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