scsi: advansys: remove #warning message

The advansys driver was converted to the proper DMA API in linux-4.2, but
the 3.18-stable kernel still warns about this:

drivers/scsi/advansys.c:71:2: warning: #warning this driver is still not properly converted to the DMA API [-Wcpp]

The warning clearly is not helpful in 3.18 any more, it just clutters up
the build log. This removes the warning instead, and clarifies the
comment above it.

Cc: Hannes Reinecke <hare@suse.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>