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>

arch/arm/cpu/armv7/mx6/clock.c

@@ -746,10 +746,11 @@ unsigned int mxc_get_clock(enum mxc_clock clk)
 	case MXC_SATA_CLK:
 		return get_ahb_clk();
 	default:
+		printf("Unsupported MXC CLK: %d\n", clk);
 		break;
 	}
 
-	return -1;
+	return 0;
 }
 
 /*