msm: msm_watchdog_v2: Correct slack time calculation

A misprint in the documentation for the WDT0_STS  register has been
corrected. Update the code to match to updated bit positions, and
also to read the register twice before accepting it as a real slack
value. This is needed because hardware does not have synchronization
between the sleep clock (used for the watchdog) and the AHB clock
(used for the read).

Change-Id: I82d2190784b7385c8805e13bf7f4e0eda0709922
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>