pkg/spidev/spidev_linux_test.go: fix invalid usage of reflect.SliceHeader
pkg/spidev/spidev_linux_test.go: fix invalid usage of reflect.SliceHeader

This is an error caught by go vet 1.17. Fixed by using a safe way
of pointing data.

Signed-off-by: Medicine Yeh <9acf644e4a244bc193f1dcb0b92021576692c463@gmail.com>

u-root/u-root,u-root/u-root,u-root/u-root