Since WE7 /proc/net/wireless checks whether level and noise  are in dBm
and shows them accordingly. Indicate that we return signal and noice
levels in dBm.

Before:
Inter-| sta-|   Quality        |   Discarded packets               | Missed | WE
 face | tus | link level noise |  nwid  crypt   frag  retry   misc | beacon | 22
  eth1: 0000   65.  219.  165.       0      0    148     41      0        0

After:
Inter-| sta-|   Quality        |   Discarded packets               | Missed | WE
 face | tus | link level noise |  nwid  crypt   frag  retry   misc | beacon | 22
  eth1: 0000   65.  -37.  -91.       0      0      0      0      0        0

While at it, replace raw numbers with appropriate macro.
Signed-off-by: NAndrey Borzenkov <arvidjaar@mail.ru>
Signed-off-by: NPavel Roskin <proski@gnu.org>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>