rtl8187: remove priv->mode

It is checked in add_interface, but there it is easily replaced with
a check of priv->vif.  If that information should become necessary,
it is available in vif->type anyway.

It is also checked in led_turn_on and led_turn_off, where I made the
substitutions as described above.  Of course, these checks seem to
have been incorrect since the driver was using NL80211_IFTYPE_MONITOR
to indicate no interface rather than NL80211_IFTYPE_UNSPECIFIED.
Anyway, I think these checks may be extraneous...?

Signed-off-by: John W. Linville <linville@tuxdriver.com>