[PATCH] ieee80211_if_set_type: make check for master dev more explicit

Problem description by Daniel Drake <dsd@gentoo.org>:

"This sequence of events causes loss of connectivity:

<plug in>
<associate as normal in managed mode>
ifconfig eth7 down
iwconfig eth7 mode monitor
ifconfig eth7 up
ifconfig eth7 down
iwconfig eth7 mode managed
<associate as normal>

At this point you are associated but TX does not work. This is because
the eth7 hard_start_xmit is still ieee80211_monitor_start_xmit."

The problem is caused by ieee80211_if_set_type checking for a non-zero
hard_start_xmit pointer value in order to avoid changing that value for
master devices.  The fix is to make that check more explicitly linked to
master devices rather than simply checking if the value has been
previously set.

CC: Daniel Drake <dsd@gentoo.org>
Acked-by: Michael Wu <flamingice@sourmilk.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>