hwmon: pm8xxx-adc: Fix reading sysfs for vbatt

The memory where the sysfs attribute is stored on
initialization gets overwritten causing invalid length
on the vbatt sysfs reading. Fix it by removing the
dynamic memory allocation for BTM parameters.

Change-Id: If8b88c9460b0018a159688b5ba28798d97e0b0b6
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>