regulator: qcom: Tidy up probe()

Tidy up error reporting and move rpm reference retrieval out of the for
loop for improved readability.

Signed-off-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Mark Brown <broonie@kernel.org>