wlcore: consider single fw case

When a single fw is being used for both single-role
and multi-role cases (e.g. 18xx), wl->mr_fw_name is
NULL, which results in NULL dereference while trying
to load the multi-role fw.

In this case, always use the single-role fw, and avoid
redundant fw switch by checking for this case in
wl12xx_need_fw_change() as well.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <luca@coelho.fi>