dmaengine: owl-dma: fix kernel-doc style for enum

Driver doesn't use keyword enum for enum owl_dmadesc_offsets resulting
in warning:

drivers/dma/owl-dma.c:139: warning: cannot understand function prototype:
'enum owl_dmadesc_offsets '

So add the keyword to fix it and also add documentation for missing
OWL_DMADESC_SIZE

Signed-off-by: Vinod Koul <vkoul@kernel.org>
Link: https://lore.kernel.org/r/20201007083113.567559-6-vkoul@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>

Author: vinodkoul

drivers/dma/owl-dma.c

@@ -124,7 +124,7 @@
 #define FCNT_VAL				0x1
 
 /**
- * owl_dmadesc_offsets - Describe DMA descriptor, hardware link
+ * enum owl_dmadesc_offsets - Describe DMA descriptor, hardware link
  * list for dma transfer
  * @OWL_DMADESC_NEXT_LLI: physical address of the next link list
  * @OWL_DMADESC_SADDR: source physical address
@@ -135,6 +135,7 @@
  * @OWL_DMADESC_CTRLA: dma_mode and linklist ctrl config
  * @OWL_DMADESC_CTRLB: interrupt config
  * @OWL_DMADESC_CONST_NUM: data for constant fill
+ * @OWL_DMADESC_SIZE: max size of this enum
  */
 enum owl_dmadesc_offsets {
 	OWL_DMADESC_NEXT_LLI = 0,