Fix endless re-transmissions when num_xfers > 1

[kodidu]

Single xfer with 2 bytes: OK mcp2210-util -D /dev/spidevX spi 9800 // SPI_IOC_MESSAGE(1)

Two xfers with 1 byte: ERROR endless transmission mcp2210-util -D /dev/spidevX spi 98,00 // SPI_IOC_MESSAGE(2)

cmd->xfer is not updated after the first xfer finished => endless transmission

Looks like commit c4f83f1fc525ac75874db9ecbdbe285a14368afe introduced this issue: https://github.com/daniel-santos/mcp2210-linux/commit/c4f83f1fc525ac75874db9ecbdbe285a14368afe#diff-0215cfa293de9d1a72422d837fa2be64c49391dfb2835b99adb31403298f0206L772

[daniel-santos]

Thank you for your patch! I'll try to review and test in the next few days.

[iDoka]

@daniel-santos any updates?

[kodidu]

@iDoka I only can suggest you to step away from the MCP2210. This is a very poorly designed communication IC...

[iDoka]

@kodidu any alternative candidate to SPI -to-USB bridge with good works on Linux?