power: qpnp-linear-charger: Allow to disable safety-timer

Safety-timer means maximum duration in minutes that a single
charge cycle may last.
Add an option to disable safety-timer when property
"qcom,tchg-mins" is set to <0>.

Change-Id: I6d56d42a58afdf82cc997bb18bbe956edfc217ae
Signed-off-by: Zhenhua Huang <zhenhuah@codeaurora.org>