SEATTLE – Brad Miller hit a pair of home runs, including his first grand slam, and the Seattle Mariners beat the Oakland Athletics 7-5 Saturday in a game that decided division-series matchups in the American League. Boston clinched home-field advantage throughout the postseason with Oakland’s loss. The A.L. West champion A’s will play their playoff opener at home next Friday against A.L. Central champion Detroit, and A.L. East winner Boston will start at Fenway Park against the team emerging from the wild-card playoff: Cleveland, Tampa Bay or Texas.