mac80211: fix dynamic-ps/pm_qos magic numbers

mac80211: fix dynamic-ps/pm_qos magic numbers

mac80211 uses pm_qos (/dev/network_latency) in order to determine the
dynamic ps timeout (or disable the dynamic-ps at all in some cases).

commit 9aa350e4 added a comparison for the current network_latency
against one high value (1900ms), and against the default value
(2000sec, rather than the commented 2sec).

however, the representation of 1900ms was incorrect:
1900ms = 1900000us ( != 1900000000 )

fix it by using USEC_TO_MSEC/SEC consts.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>