[flashrom] SPIPGM bug for SST25*

[Carl-Daniel Hailfinger]

Hi RayeR,

I found that SST25 flash only allows one-byte write and no page write.
But it uses the normal page write opcode.
To fix this, SPI_FLASH_PAGE_SIZE must be 1 instead of 256 for SST25 family.

Regards,
Carl-Daniel