Staging: rtl8192u: use correct array for debug output

This is supposed to be ->rates_ex[] instead of ->rates[].  I found this
because static checkers complain than ->rates is too small so we're
reading beyond the end of the array.  It has 12 elements instead of 15.

This bug was apparently copy and pasted from ipw2x00.  I fixed it before
in that driver 428e3cf5 ('ipw2x00: printing the wrong array in
debug code')

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>