ata: ahci: Cleanup ahci_reset_controller()

Fix multi-line comment style in ahci_reset_controller() and change the
code to return early if ahci_skip_host_reset is true, reducing
indentation by one level for the bulk of the function code.

No functional changes.

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