ARM: OMAP2+: Variable "reg" in function omap4_dsi_mux_pads() could be uninitialized

In function omap4_dsi_mux_pads(), local variable "reg" could
be uninitialized if function regmap_read() returns -EINVAL.
However, it will be used directly in the later context, which
is potentially unsafe.

Signed-off-by: Yizhuo <yzhai003@ucr.edu>
Signed-off-by: Tony Lindgren <tony@atomide.com>