staging: r8188eu: refactor validate_recv_frame's error handling

Refactor the error handling of validate_recv_frame. It's simpler to
default to _FAIL.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20220227164147.1168847-4-martin@kaiser.cx


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