mmc: am654_sdhci: Unconditionally switch off DLL in the beginning of ios_post()

There are some speed modes that work without switching the dll on.
Unconditionally switch off the DLL before setting clock frequency to
support this case. The software will automatically enable DLL for speed
modes that require it. This also means the dll_on priv data member is no
longer required.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Signed-off-by: Aswath Govindraju <a-govindraju@ti.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>