msm: qpnp: Fix stub definition

Fix the stub definition of below functions to avoid compilation
failures if the SPMI driver is disabled.

qpnp_vadc_calib_vref()
qpnp_vadc_calib_gnd()
qpnp_adc_disable_voltage()
qpnp_adc_free_voltage_resource()

Change-Id: I23479a51b3ef0342a0505fca94ddd856768101e5
Signed-off-by: Srinivas Ramana <sramana@codeaurora.org>