mfd: qcom-spmi-pmic: Don't access non-existing registers

Revision ID registers are available only on devices with
Slave IDs that are even, so don't make access to unavailable
registers.

Signed-off-by: Ivan T. Ivanov <ivan.ivanov@linaro.org>
[sboyd@codeaurora.org: Consider all slave ids that are even]
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>