ath9k: Fix bug in reading debugfs file 'rcstat'

The rate table would not have been chosen before the interface
has been brought up. Reading 'rcstat' in this case would result
in an oops, fix this.

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