pcmcia: add pcmcia_loop_config() helper

By calling pcmcia_loop_config(), a pcmcia driver can iterate over all
available configuration options. During a driver's probe() phase, one
doesn't need to use pcmcia_get_{first,next}_tuple, pcmcia_get_tuple_data
and pcmcia_parse_tuple directly in most if not all cases.

Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>