support xfer->cs_change in multi-part SPI messages

[LouisLambda]

Hi, thanks for this driver. It's been really useful.

I looked at the stock transfer_one_message implementation from the kernel's [spi.c], and it explicitly handles xfer->cs_change which the this driver currently doesn't. cs_change is meant to tell the SPI driver to momentarily toggle the CS mid-message, since some chips require that to "commit" a command.

[frank-zago]

Could you look at that PR: #16 ?

I didn't find a device whose driver generate a multipart message, so I can't really test. I always see cs_change==0.

If cs_change is not set, and there is a next message, then keep the CS line up.