serial_cs: use pcmcia_loop_config() and pre-determined values

As the PCMCIA core already determines the multifunction count, the
ConfigBase address and the Present value, we can use them directly
instead of parsing the CIS again. By making use of pcmcia_loop_config(),
we can further remove the remaining call to pcmcia_get_first_tuple()
and friends.

CC: linux-serial@vger.kernel.org
CC: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>