async_tx: fix kernel-doc notation warnings

Fix kernel-doc warnings by adding "struct" keyword or "enum" keyword.
Also fix 2 function parameter descriptions.
Change some functions and structs from kernel-doc /** notation
to regular /* comment notation.

async_pq.c:18: warning: cannot understand function prototype: 'struct page *pq_scribble_page; '
async_pq.c:18: error: Cannot parse struct or union!
async_pq.c:40: warning: No description found for return value of 'do_async_gen_syndrome'
async_pq.c:109: warning: Function parameter or member 'blocks' not described in 'do_sync_gen_syndrome'
async_pq.c:109: warning: Function parameter or member 'offsets' not described in 'do_sync_gen_syndrome'
async_pq.c:109: warning: Function parameter or member 'disks' not described in 'do_sync_gen_syndrome'
async_pq.c:109: warning: Function parameter or member 'len' not described in 'do_sync_gen_syndrome'
async_pq.c:109: warning: Function parameter or member 'submit' not described in 'do_sync_gen_syndrome'

async_tx.c:136: warning: cannot understand function prototype: 'enum submit_disposition '
async_tx.c:264: warning: Function parameter or member 'tx' not described in 'async_tx_quiesce'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: linux-crypto@vger.kernel.org
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>