Minor tweaks.

- Rename the nfc_device_t's struct iErrorCode member to iPICCError (We are
    likely to have both PICC and PCD errors fields to avoid unneeded complexity
    at some point);
  - Make the PN53x error descriptions static;
  - Enhance some comments here and there.

include/nfc/nfc-types.h

@@ -69,8 +69,8 @@ typedef struct {
   bool bPar;
 /** The last tx bits setting, we need to reset this if it does not apply anymore */
   uint8_t ui8TxBits;
-/** The last error encountered by the device */
-  int iErrorCode;
+/** Last error reported by the PICC */
+  int iPICCError;
 } nfc_device_t;