ata: libata-core: Remove ata_port_suspend_async()

ata_port_suspend_async() is only called by ata_sas_port_suspend().
Modify ata_port_suspend() with an additional bool argument indicating an
asynchronous or synchronous suspend to allow removing that helper
function. With this change, the variable ata_port_resume_ehi can also be
removed and its value (ATA_EHI_XXX flags passed directly to
ata_port_request_pm().

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Tested-by: Chia-Lin Kao (AceLan) <acelan.kao@canonical.com>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>