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
Linux 3.0.97
Linux 3.0.98
Linux 3.0.99