Fix wrong usage of iwx_lookup_cmd_ver() in iwx_send_soc_conf().

Fix wrong usage of iwx_lookup_cmd_ver() in iwx_send_soc_conf().

Callers are supposed to check whether iwx_lookup_cmd_ver() returns
CMD_VER_UNKNOWN, and this check was missing here. Fortunately, the
buggy check was part of a condition which also requires the
low_latency_xtal constant to be set. We do not yet support devices
where low_latency_xtal is non-zero, so the bug never triggered.