imx:mx6 fix return value of mxc_get_clock

imx:mx6 fix return value of mxc_get_clock

mxc_get_clock's return type is unsigned int. 'return -1' is same with
'return 0xffffffff', so 0 should be used as the return value when
unsupported mxc_clock type is passed to mxc_get_clock.

Also include an err message when unsupported mxc_clock type is passed
to mxc_get_clock.

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>
Reviewed-by: Fabio Estevam <fabio.estevam@freescale.com>