[patch] use unsigned char in smc91111 driver for mac

the v_mac variable in the smc91111 driver is declared as a signed char ...
this causes problems when one of the bytes in the MAC is "signed" like 0xE0
because when it gets printed out, you get a display like:
0xFFFFFFE0 and that's no good

Signed-off-by: Mike Frysinger <vapier@gentoo.org>