Fix DMAChannel transferSize() on Teensy LC

fixes #266

teensy3/DMAChannel.h

 	void transferSize(unsigned int len) {
 		uint32_t dcr = CFG->DCR & 0xF0C8FFFF;
 		if (len == 4) {
-			CFG->DCR = dcr | DMA_DCR_DSIZE(0) | DMA_DCR_DSIZE(0);
+			CFG->DCR = dcr | DMA_DCR_SSIZE(0) | DMA_DCR_DSIZE(0);
 		} else if (len == 2) {
-			CFG->DCR = dcr | DMA_DCR_DSIZE(2) | DMA_DCR_DSIZE(2);
+			CFG->DCR = dcr | DMA_DCR_SSIZE(2) | DMA_DCR_DSIZE(2);
 		} else {
-			CFG->DCR = dcr | DMA_DCR_DSIZE(1) | DMA_DCR_DSIZE(1);
+			CFG->DCR = dcr | DMA_DCR_SSIZE(1) | DMA_DCR_DSIZE(1);
 		}
 	}