pcmcia: check for pointer instead of pointer address

Bug noted by Michael Buesch: checking for the pointer address is always true.
This didn't matter much, for the very first check in pcmcia_release_window()
was for the pointer pointing to something, and the return value is ignored
here. Nonetheless, fix it.

CC: Michael Buesch <mb@bu3sch.de>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>