m68k/amiga: Use PTR_RET rather than if(IS_ERR(...)) + PTR_ERR

The semantic patch that makes this change is available
in scripts/coccinelle/api/ptr_ret.cocci.

More information about semantic patching is available at
http://coccinelle.lip6.fr/



Signed-off-by: Thomas Meyer <thomas@m3y3r.de>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>