regulator: spm-regulator: use debug log level for SPMI fall back message

The msm_spm_set_vdd() function can only succeed if at least one
core of the cluster powered by the supply regulator is online.
However, it is valid for consumers to make voltage requests when
all cores are offline.  This typically happens during suspend
after the last core of a secondary cluster is taken offline or
during resume before the first core of a secondary cluster is
brought back online.

Change the log level for the SPMI fall back message that is
printed when msm_spm_set_vdd() returns an error from err to
debug.  This ensures that confusion cannot arise from seeing
the fall back message and assuming that it was generated due
to incorrect operation.

Change-Id: I5a970173f628c762b0ec328aa8a6ba2c4bc1bdf1
Signed-off-by: David Collins <collinsd@codeaurora.org>