agp/ati: Return error from ati_create_page_map()

Fix the compiler warning

  drivers/char/agp/ati-agp.c: In function 'ati_create_page_map':
  drivers/char/agp/ati-agp.c:58:16: warning: variable 'err' set but not used [-Wunused-but-set-variable]
    58 |         int i, err = 0;

by returing the error to the caller.

v2:
	* free page in error branch (Helge)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Helge Deller <deller@gmx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20211201114645.15384-5-tzimmermann@suse.de