Sends a command to the serial
flash through the SPI.
The command is made up of two parts: the first is used to transmit the command byte and optionally, address and dummy bytes. The second part is the data to send or receive. This function does not block: it returns as soon as the transfer has been started. An optional callback
can be invoked to notify the end of transfer. Return 0 if successful; otherwise, returns
AT26_ERROR_BUSY if the AT26 driver is currently executing a command, or
AT26_ERROR_SPI if the command cannot be sent because of a SPI error.