ata: Only output sg element mapped number in verbose debug

Defining `ATA_DEBUG` there are a lof of messages like below in the log.

    [   16.345472] ata_sg_setup: 1 sg elements mapped

As that is too verbose, only output these messages in verbose debug.

Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Tejun Heo <tj@kernel.org>