add csChange, pad fields to spiIOCTransfer struct

Per the discussion in #38, these two fields are part of the equivalent
struct in the kernel SPI driver. Adding the fields allows the SPI driver
to work on a Raspberry Pi Model 2 B.

h/t @hbhasker

Fixes #38. Fixes #24.

host/generic/spibus.go

@@ -36,6 +36,9 @@ type spiIOCTransfer struct {
 	speedHz     uint32
 	delayus     uint16
 	bitsPerWord uint8
+
+	csChange uint8
+	pad      uint32
 }
 
 type spiBus struct {