regulator: spm-regulator: initialize SAW VCTL register at probe time

In order for msm_spm_get_vdd() to return a valid voltage level that
can be used by consumers, an msm_spm_set_vdd() call must take place
beforehand. To cater to this scenario, perform an msm_spm_set_vdd()
call with the regulator supply voltage read via the SPMI bus at
initialization time.

Change-Id: Ic11072b54728f9bcac91f306d2957f3934c1e448
Signed-off-by: Osvaldo Banuelos <osvaldob@codeaurora.org>