mtd: spi-nor: intel: use ERR_CAST in return statement

This fixes a sparse warning about incorrect type in return expression.

Signed-off-by: Nicholas Mc Guire <der.herr@hofr.at>
Acked-by: Marek Vasut <marek.vasut@gmail.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>