wifi: wfx: relocate wfx_rate_mask_to_hw()

wfx_rate_mask_to_hw() is only used in hif_tx.c. So relocate it into
hif_tx.c and mark it static.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20231004172843.195332-3-jerome.pouiller@silabs.com