Do not access `sc' directly via the CSR_{READ,WRITE}_[124] macros.

Noticed and patch provided by Amol via PR kern/51708.

Index: if_alcreg.h
===================================================================
RCS file: /cvsroot/src/sys/dev/pci/if_alcreg.h,v
retrieving revision 1.5
diff -u -p -r1.5 if_alcreg.h
--- if_alcreg.h	24 Nov 2015 23:30:04 -0000	1.5
+++ if_alcreg.h	27 Dec 2016 09:52:48 -0000
@@ -1488,15 +1488,15 @@ struct alc_softc {
 
 /* Register access macros. */
 #define	CSR_WRITE_4(_sc, reg, val)	\
-	bus_space_write_4((sc)->sc_mem_bt, (sc)->sc_mem_bh, (reg), (val))
+	bus_space_write_4((_sc)->sc_mem_bt, (_sc)->sc_mem_bh, (reg), (val))
 #define	CSR_WRITE_2(_sc, reg, val)	\
-	bus_space_write_2((sc)->sc_mem_bt, (sc)->sc_mem_bh, (reg), (val))
+	bus_space_write_2((_sc)->sc_mem_bt, (_sc)->sc_mem_bh, (reg), (val))
 #define	CSR_WRITE_1(_sc, reg, val)	\
-	bus_space_write_1((sc)->sc_mem_bt, (sc)->sc_mem_bh, (reg), (val))
+	bus_space_write_1((_sc)->sc_mem_bt, (_sc)->sc_mem_bh, (reg), (val))
 #define	CSR_READ_2(_sc, reg)		\
-	bus_space_read_2((sc)->sc_mem_bt, (sc)->sc_mem_bh, (reg))
+	bus_space_read_2((_sc)->sc_mem_bt, (_sc)->sc_mem_bh, (reg))
 #define	CSR_READ_4(_sc, reg)		\
-	bus_space_read_4((sc)->sc_mem_bt, (sc)->sc_mem_bh, (reg))
+	bus_space_read_4((_sc)->sc_mem_bt, (_sc)->sc_mem_bh, (reg))
 
 #define	ALC_RXCHAIN_RESET(_sc)						\
 do {									\