Silabs: SPI DMA fix

DMA tranfer was not working in Geckos using DMA. Changed transfer
length calculation in function spi_activate_dma.

Author: jhokajar

libraries/mbed/targets/hal/TARGET_Silicon_Labs/TARGET_EFM32/spi_api.c

@@ -1006,7 +1006,7 @@ static void spi_activate_dma(spi_t *obj, void* rxdata, const void* txdata, int t
                             false,
                             (obj->spi.bits <= 8 ? (void *)&(obj->spi.spi->TXDATA) : (void *)&(obj->spi.spi->TXDOUBLE)), //When frame size > 9, point to TXDOUBLE
                             (txdata == 0 ? &fill_word : (void *)txdata), // When there is nothing to transmit, point to static fill word
-                            (obj->spi.bits <= 8 ? tx_length - 1 : (tx_length / 2) - 1)); // When using TXDOUBLE, recalculate transfer length
+                            (tx_length - 1));
     } else {
         /* Frame size == 9 */
         /* Only activate RX DMA if a receive buffer is specified */
@@ -1044,7 +1044,7 @@ static void spi_activate_dma(spi_t *obj, void* rxdata, const void* txdata, int t
                             false,
                             (void *)&(obj->spi.spi->TXDATAX), //When frame size > 9, point to TXDOUBLE
                             (txdata == 0 ? &fill_word : (void *)txdata), // When there is nothing to transmit, point to static fill word
-                            (tx_length / 2) - 1); // When using TXDOUBLE, recalculate transfer length
+                            (tx_length - 1));
     }
 }
 #endif //LDMA_PRESENT