pcmcia: honor saved flags in yenta_socket's I365_CSCINT register

Instead of overwriting the I365_CSCINT register, save the old value and
merely change the bits we care about.

Part 1 of a series to allow the ISA irq to be used for Cardbus devices
if the socket's PCI irq is unusable.

[linux@dominikbrodowski.net: split up the original patch, commit message]

Signed-off-by: Jens Kuenzer <Jens.Kuenzer@fpga.homeip.net>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>