interconnect: Check for valid path in icc_set_bw()

Use IS_ERR() to ensure that the path passed to icc_set_bw() is valid.

Reviewed-by: Evan Green <evgreen@chromium.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Georgi Djakov <georgi.djakov@linaro.org>