staging: wfx: fix use of uninitialized pointer

With -Wuninitialized, the compiler complains:

drivers/staging/wfx/data_tx.c:34:19: warning: variable 'band' is uninitialized when used here [-Wuninitialized]
    if (rate->idx >= band->n_bitrates) {
                         ^~~~

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Nathan Chancellor <natechancellor@gmail.com>
Fixes: 868fd970

 ("staging: wfx: improve robustness of wfx_get_hw_rate()")
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Link: https://lore.kernel.org/r/20201019160604.1609180-1-Jerome.Pouiller@silabs.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>