ipw2x00: remove the right /proc/net entry

Commit 27ae60f8 ("ipw2x00: replace "ieee80211" with "libipw" where
appropriate") changed DRV_NAME to be "libipw", but didn't properly fix
up the places where it was used to specify the name for the /proc/net/
directory.

For backwards compatibility reasons, that directory name remained
"ieee80211", but due to the DRV_NAME change, the error case printouts
and the cleanup functions now used "libipw" instead.  Which made it all
fail badly.

For example, on module unload as reported by Randy:

  WARNING: at fs/proc/generic.c:816 remove_proc_entry+0x156/0x35e()
  name 'libipw'

because it's trying to unregister a /proc directory that obviously
doesn't even exist.

Clean it all up to use DRV_PROCNAME for the actual /proc directory name.

Reported-and-tested-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Pavel Roskin <proski@gnu.org>
Cc: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>