ata: libata-eh: Rename hipm and dipm variables

Rename the hipm and dipm variables to have a clearer name.
Also fold in the usage of no_dipm, as that is required in order to give
the dipm variable a more descriptive name.

No functional change.

Signed-off-by: Niklas Cassel <cassel@kernel.org>
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>