dma: fix comments

Correct spelling typo in drivers/dma.

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
diff --git a/drivers/dma/ep93xx_dma.c b/drivers/dma/ep93xx_dma.c
index c64917e..bb02fd9 100644
--- a/drivers/dma/ep93xx_dma.c
+++ b/drivers/dma/ep93xx_dma.c
@@ -1118,7 +1118,7 @@
  * @chan: channel
  * @dma_addr: DMA mapped address of the buffer
  * @buf_len: length of the buffer (in bytes)
- * @period_len: lenght of a single period
+ * @period_len: length of a single period
  * @dir: direction of the operation
  * @context: operation context (ignored)
  *