s3an_nor_scan() was overriding the opcode selection done in
spi_nor_default_setup(). Set nor->setup() method in order to
avoid the unnecessary call to spi_nor_default_setup().
Now that the call to spi_nor_default_setup() is skipped, set
mtd.erasesize to nor->info->sector_size, as it was when
spi_nor_select_erase() was called.
No dummy byte is required for the S3AN's Random Read command (0x03),
so no need to set nor->read_dummy.