remoteproc: qcom: q6v5-mss: fixup q6v5_pds_enable error handling

dev_pm_domain_attach_by_name will return NULL if the requested
power-domain is not a part device node. This could result in
NULL pointer de-reference in q6v5_pds_enable. Fix this by
checking for IS_ERR_OR_NULL and forward the appropriate error
code.

Fixes: 4760a896 ("remoteproc: q6v5-mss: Vote for rpmh power domains")
Signed-off-by: Sibi Sankar <sibis@codeaurora.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>