net: wireless: bcmdhd: Reduce keep-alive period to 28 sec

Bug: 7386287

Change-Id: I115135e1d2cba0135c5201b95a2599dcabac0247
Signed-off-by: Dmitry Shmidt <dimitrysh@google.com>

Conflicts:

	drivers/net/wireless/bcmdhd/dhd.h
time: cast ->raw_interval to u64 to avoid shift overflow

We fixed a bunch of integer overflows in timekeeping code during the 3.6
cycle.  I did an audit based on that and found this potential overflow.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
backported from Linux 3.6+ to Tuna Hybrid kernel by faux123