NFC: NCI: zero struct spi_transfer variables before usage

Using ARM compiler, and without zero-ing spi_transfer, spi-s3c64xx
driver would issue abnormal errors due to bpw field value being set to
unexpected value. This structure MUST be set to all zeros except for
those field specifically used.

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