hwmon: qpnp-adc: Update conversion timeout value

Few adc channels fail to complete conversion with the current time out
value. Increase time out such that adc conversion is complete and correct
value is reported.

Change-Id: I11a2b44b4f2eb9ed8589d6432324e7b8ffc4a2dc
Signed-off-by: Rama Krishna Phani A <rphani@codeaurora.org>