libata: factor out a ata_identify_page_supported helper

tj: Updated line continuation style for consistency as pointed out by
    Sergei.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Cc: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Tejun Heo <tj@kernel.org>