NFC: digital: Fix incorrect use of ERR_PTR and PTR_ERR macros

It's bad to use these macros when not dealing with error code. this
patch changes calls to these macros with correct casts.

Signed-off-by: Thierry Escande <thierry.escande@linux.intel.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>