ata: brcm: fix reset controller API usage

While fixing another issue in this driver I noticed it uses
IS_ERR_OR_NULL(), which is almost always a mistake.

Change the driver to use the proper devm_reset_control_get_optional()
interface instead and remove the checks except for the one that
checks for a failure in that function.

Fixes: 2b2c47d9 ("ata: ahci_brcm: Allow optional reset controller to be used")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>