staging: vt6656: Use define instead of magic number for tx_rate

Use the define RATE_11M present in the file "device.h" instead of the
magic number 3. So the code is more clear.

Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Oscar Carter <oscar.carter@gmx.com>
Link: https://lore.kernel.org/r/20200414153849.5785-4-oscar.carter@gmx.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>