wifi: mwifiex: remove unnecessary queue empty check

Since Commit 7bff9c97 ("mwifiex: send firmware initialization commands
synchronously") all initialization commands are sent synchronously which
means the command queue is empty when mwifiex_sta_init_cmd() returns. No
need to check for entries in the command code then, so remove the check.

Add a WARN_ON() just in case there is something wrong with the
reasoning.

Reviewed-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Acked-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Link: https://patch.msgid.link/20250423-mwifiex-drop-asynchronous-init-v2-1-1bb951073a06@pengutronix.de


[remove now unused is_cmd_pend_q_empty variable and comment]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>