pcmcia: move some window-related code to pcmcia_ioctl.c

pcmcia_get_window() and pcmcia_get_mem_page() were only called from
pcmcia_ioctl.c. Therefore, move these functions to that file, and
remove the useless EXPORTs.

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