Fix wrong var names in PICC_Select, MFRC522.cpp (#301)

fix issue #238

src/MFRC522.cpp

@@ -736,10 +736,10 @@ MFRC522::StatusCode MFRC522::PICC_Select(	Uid *uid,			///< Pointer to Uid struct
 					return STATUS_INTERNAL_ERROR;
 				}
 				// Choose the PICC with the bit set.
-				currentLevelKnownBits = collisionPos;
-				count			= (currentLevelKnownBits - 1) % 8; // The bit to modify
-				index			= 1 + (currentLevelKnownBits / 8) + (count ? 1 : 0); // First byte is index 0.
+				count			= (collisionPos - 1) % 8; // The bit to modify
+				index			= 1 + ((collisionPos - 1) / 8) + (count ? 1 : 0); // First byte is index 0.
 				buffer[index]	|= (1 << count);
+				currentLevelKnownBits = collisionPos;
 			}
 			else if (result != STATUS_OK) {
 				return result;