S3AN flashes use a specific opcode to read the status register.
We currently use the SPI_S3AN flag to decide whether this specific
SR read opcode should be used, but SPI_S3AN is about to disappear, so
let's add a new flag.
Note that we use the same bit as SPI_S3AN implies SPI_NOR_XSR_RDY and
vice versa.