net: pxa168_eth: avoid using signed char for bitops

Signedness bugs may occur when using signed char for bitops,
depending on if the highest bit is ever used.

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>