The LD1585 will only do for a small('ish) motor.
It can handle around 2W un-sinked and with a large heat sink around 25W tops.
Regulating it to eg. 6V will make it drop 6V (actually a bit more, since the terminal voltage is more than 12V).
Un-sinked this equates to 2W/6V=333 mA
With perfect heat sinking it will be 25W/6V = 4,17A
Regulating the output further down will only worsen this to around 166 mA and 2,1A respectively.
(It's all in the datasheet).