ata: libata: Cleanup inline DMA helper functions

Simplify the inline DMA helper functions ata_using_mwdma(),
ata_using_udma() and ata_dma_enabled() to directly return as a boolean
the result of their test condition.

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>