media: camss-vfe: always initialize reg at vfe_set_xbar_cfg()

if output->wm_num is bigger than 2, the value for reg is
not initialized, as warned by smatch:
	drivers/media/platform/qcom/camss-8x16/camss-vfe.c:633 vfe_set_xbar_cfg() error: uninitialized symbol 'reg'.
	drivers/media/platform/qcom/camss-8x16/camss-vfe.c:637 vfe_set_xbar_cfg() error: uninitialized symbol 'reg'.

That shouldn't happen in practice, so add a logic that will
break the loop if i > 1, fixing the warnings.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Acked-by: Todor Tomov <todor.tomov@linaro.org>