caif: Bugfix for socket priority, bindtodev and dbg channel.

Changes:
o Bugfix: SO_PRIORITY for SOL_SOCKET could not be handled
  in caif's setsockopt,  using the struct sock attribute priority instead.

o Bugfix: SO_BINDTODEVICE for SOL_SOCKET could not be handled
  in caif's setsockopt,  using the struct sock attribute ifindex instead.

o Wrong assert statement for RFM layer segmentation.

o CAIF Debug channels was not working over SPI, caif_payload_info
  containing padding info must be initialized.

o Check on pointer before dereferencing when unregister dev in caif_dev.c

Signed-off-by: Sjur Braendeland <sjur.brandeland@stericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>