wusbcore: reduce keepalive threshold from timeout/2 to timeout/3

This patch reduces the keepalive threshold of WUSB host controllers from
timeout/2 to timeout/3.  The keepalive timer fires every timeout/2 ms, but
due to rounding errors and jitter, the host may decide not to send a
keepalive at timeout/2.  By the time the next timer fires, a full timeout
period may have expired causing the device to be disconnected without ever
having been sent a keepalive.  Changing the keepalive threshold to
timeout/3 ensures that at least one keepalive will be sent before a device
is disconnected.  The patch also updates the code to use msecs_to_jiffies
consistently.

Signed-off-by: Thomas Pugliese <thomas.pugliese@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>