NFC: NCI: Store the spi device pointer from the spi instance

Storing the spi device was forgotten in the original implementation,
which would pretty obviously cause some kind of serious crash when
actually trying to send something through that device.

Signed-off-by: Eric Lapuyade <eric.lapuyade@intel.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>