staging: wilc1000: move current_tx_rate enum to wilc_wlan_if.h file

Move 'current_tx_rate' enum to wilc_wlan_if.h, to have it along with the
other FW related to constants. Also added prefix to have a better
namespace.

Signed-off-by: Ajay Singh <ajay.kathat@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>