staging: rtl8723bs: refactor rtw_joinbss_event_prehandle to reduce indentation

staging: rtl8723bs: refactor rtw_joinbss_event_prehandle to reduce indentation

The rtw_joinbss_event_prehandle function has excessive indentation due
to deeply nested if-statements.

Refactor the function using early returns and guard clauses for the
failure paths. This flattens the code and significantly improves
readability.

Signed-off-by: Jose A. Perez de Azpillaga <azpijr@gmail.com>
Reviewed-by: Luka Gejak <luka.gejak@linux.dev>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://patch.msgid.link/20260321182713.665872-3-azpijr@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>