ARM: imx: fix register offset of pll7_usb_host gate clock

There is a copy&paste error on register offset of pll7_usb_host gate
clock introduced by i.MX6 PLL bypass support patches.  The error breaks
the ENET function, because it overwrites the pll6_enet gate bit.

Correct the offset for all i.MX6 clock drivers.

Thanks to Fugang Duan <B38611@freescale.com> for spotting the error.

Signed-off-by: Shawn Guo <shawn.guo@freescale.com>