regulator: spm-regulator: check spm status in probe function

Check the status of the spm driver using msm_spm_probe_done()
inside of the spm-regulator probe function.  Return any error
code from the probe function.  This ensures that spm-regulator
will defer its probe in the case that the spm driver has not
yet probed.

Change-Id: I6d006cee6289bd51e38271e70f81c7a4622016ee
Signed-off-by: David Collins <collinsd@codeaurora.org>