smb-lib: allow reading vbus without presence

Currently the driver skips invoking adc api's if usb is not
present i.e. vbus has fallen below 3.3V.

This presence check is not required for the vbus. Skip that
check.

Change-Id: I4a2dd91109a308b553b7407618ee8f3221127f61
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>