fdt: remove unaligned access in fdt_fixup_ethernet()

Some ARM compilers may emit code that makes unaligned accesses when
faced with constructs such as:

char mac[16] = "ethaddr";

Replace this with a strcpy() call instead to avoid this. strcpy() is
used here, rather than replacing all usage of the mac variable with the
string itself, since the loop itself sprintf()s to the variable each
iteration, so strcpy() is doing basically the same thing.

Reported-by: Florian Meier
Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>