err.h: move IOMEM_ERR_PTR() to err.h

commit 18311a76 upstream.

Since IOMEM_ERR_PTR() macro deals with an error pointer, a better place
for it is err.h. This helps avoid dependency on io.h for the users that
don't need it.

Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Raag Jadav <raag.jadav@intel.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>