staging: rtl8192e: remove CURRENT_RATE macro

CURRENT_RATE macro is used only once in driver. This patch removes it
and adds rtllib_current_rate() static function in file using that macro

Signed-off-by: Mateusz Kulikowski <mateusz.kulikowski@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>