NFC: pn533: Add MI/TG bits only when in Initiator mode

The fragmentation routine (used to split big frames) could be used in
target or initiator mode (TgSetMetaData vs InDataExchange), but the
MI/TG bytes are not needed in target mode (TgSetMetaData), so we
add a check on the mode

Signed-off-by: Olivier Guiter <olivier.guiter@linux.intel.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>