nl80211: remove some stack variables in trigger_scan and start_sched_scan

Some stack variables (name *ssid and *channel) are only used to define
the size of the memory block that needs to be allocated for the
request structure in the nl80211_trigger_scan() and
nl80211_start_sched_scan() functions.

This is unnecessary because the sizes of the actual elements in the
structure can be used instead.

Signed-off-by: Luciano Coelho <coelho@ti.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>