myx(4): add initialization of sc_sff_lock rwlock

[Kevin Lo]

Ok?

Index: sys/dev/pci/if_myx.c
===================================================================
RCS file: /cvs/src/sys/dev/pci/if_myx.c,v
retrieving revision 1.111
diff -u -p -u -p -r1.111 if_myx.c
--- sys/dev/pci/if_myx.c        17 Jul 2020 03:37:36 -0000      1.111
+++ sys/dev/pci/if_myx.c        26 Nov 2020 06:49:04 -0000
@@ -270,6 +270,8 @@ myx_attach(struct device *parent, struct
        char                     part[32];
        pcireg_t                 memtype;
 
+       rw_init(&sc->sc_sff_lock, "myxsff");
+
        sc->sc_pc = pa->pa_pc;
        sc->sc_tag = pa->pa_tag;
        sc->sc_dmat = pa->pa_dmat;