fix linker errors when compiling with clang

[dotnixd]

[dotnixd]

log (without fix):

make ARCH=x86_64 CROSS_COMPILE= -C /lib/modules/5.4.44/build M=/home/unix/Sources/rtl8821ce-tomaspinho  modules
make[1]: Entering directory '/usr/src/linux-5.4.44'
    Building modules, stage 2.
    MODPOST 1 modules.
ERROR: "is_zero_mac_addr" [/home/unix/Sources/rtl8821ce-tomaspinho/8821ce.ko] undefined!
ERROR: "is_multicast_mac_addr" [/home/unix/Sources/rtl8821ce-tomaspinho/8821ce.ko] undefined!
ERROR: "is_broadcast_mac_addr" [/home/unix/Sources/rtl8821ce-tomaspinho/8821ce.ko] undefined!
make[2]: *** [scripts/Makefile.modpost:94: __modpost] Error 1
make[1]: *** [Makefile:1616: modules] Error 2
make[1] Leaving directory '/usr/src/linux-5.4.44'
make: *** [Makefile:2244: modules] Error 2