driver core: remove __must_check from device_create_file

With the conversion of the sysdev to a real struct device, more drivers
are calling device_create_file, and some of them don't check the return
value, which isn't wise.

But as they happen to be in parts of the kernel where a warning is
considered an error (i.e. powerpc), this breaks the build.  So for now,
remove the marking on the function, which fixes the build problems.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>