clk: imx: clk-pllv3: fix incorrect handle of enet powerdown bit

After commit f5394745 ("ARM: clk: imx: update pllv3 to support imx7"),
the former used BM_PLL_POWER bit is not correct anymore for IMX7 ENET.
Instead, pll->powerdown holds the correct bit, so using powerdown bit
in clk_pllv3_{prepare | unprepare} functions.

Fixes: f5394745

 ("ARM: clk: imx: update pllv3 to support imx7")
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>