net/macsonic: Drop redundant MACH_IS_MAC test

The MACH_IS_MAC test is redundant here because the platform device
won't get registered unless MACH_IS_MAC.

Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: David S. Miller <davem@davemloft.net>