wifi: mwifiex: deduplicate code in mwifiex_cmd_tx_rate_cfg()

wifi: mwifiex: deduplicate code in mwifiex_cmd_tx_rate_cfg()

The code block inside the if/else is the same with just using
pbitmap_rates if non NULL or priv->bitmap_rates otherwise. Deduplicate
the code by picking the correct pointer first and then using it
unconditionally.

Reviewed-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Brian Norris <briannorris@chromium.org>
Link: https://patch.msgid.link/20250410-mwifiex-cleanup-1-v6-1-a6bbd4ac4d37@pengutronix.de
Signed-off-by: Johannes Berg <johannes.berg@intel.com>