staging: wfx: split out wfx_tx_fill_rates() from wfx_tx_confirm_cb()

wfx_tx_confirm_cb() is a big function. A big part of its body aims to
fill the rates list. So, create a new function wfx_tx_fill_rates() and
make wfx_tx_confirm_cb() smaller.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200515083325.378539-9-Jerome.Pouiller@silabs.com


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