mtd: mchp23k256: propagate return value of spi_sync()

The call to spi_sync() can fail.
Check the return value and propagate it.

Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>