scsi: ufshcd: Allow zero value setting to Auto-Hibernate Timer

The zero value Auto-Hibernate Timer is a valid setting, and it
indicates the Auto-Hibernate feature being disabled. Correctly
support this setting. In addition, when this value is queried
from sysfs, read from the host controller's register and return
that value instead of using the RAM value.

Change-Id: I6ad0e00ae485549cb18f704907149d8a69961e5f
Signed-off-by: Bao D. Nguyen <nguyenb@codeaurora.org>