NFC: pn533: Add frame header length define

PN533_CMD_DATAEXCH_HEAD_LEN includes a frame header length which is not
seen at a glance. It can be missleading, so split it and define the
frame header length explicitly.

Signed-off-by: Waldemar Rymarkiewicz <waldemar.rymarkiewicz@tieto.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>