mtd: spi-nor: fix error handling in spi_nor_erase

The documenting comment of mtd_erase in mtdcore.c states:
Device drivers are supposed to call instr->callback() whenever
the operation completes, even if it completes with a failure.

Currently the callback isn't called in case of failure. Fix this.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>