mtd: spi-nor: handle timeout errors in spi_nor_write()

The error label was unused here. It looks like we're missing at least
one case that should be doing 'goto write_err'.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Acked-by: Huang Shijie <shijie8@gmail.com>