NFC: digital: Fix a possible memory leak

This fixes a memory leak issue that may occur if data sending fails in
initiator mode. The data_exch structure was not released in case of
error.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Thierry Escande <thierry.escande@linux.intel.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>