net: ax88796c: Fix clang -Wimplicit-fallthrough in ax88796c_set_mac()

Clang warns:

drivers/net/ethernet/asix/ax88796c_main.c:696:2: error: unannotated fall-through between switch labels [-Werror,-Wimplicit-fallthrough]
        case SPEED_10:
        ^
drivers/net/ethernet/asix/ax88796c_main.c:696:2: note: insert 'break;' to avoid fall-through
        case SPEED_10:
        ^
        break;
drivers/net/ethernet/asix/ax88796c_main.c:706:2: error: unannotated fall-through between switch labels [-Werror,-Wimplicit-fallthrough]
        case DUPLEX_HALF:
        ^
drivers/net/ethernet/asix/ax88796c_main.c:706:2: note: insert 'break;' to avoid fall-through
        case DUPLEX_HALF:
        ^
        break;

Clang is a little more pedantic than GCC, which permits implicit
fallthroughs to cases that contain just break or return. Clang's version
is more in line with the kernel's own stance in deprecated.rst, which
states that all switch/case blocks must end in either break,
fallthrough, continue, goto, or return. Add the missing breaks to fix
the warning.

Link: https://github.com/ClangBuiltLinux/linux/issues/1491


Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>