depca: fix warning in drivers/net/depca.c

fix warning:

  drivers/net/depca.c: In function ‘depca_eisa_probe’:
  drivers/net/depca.c:1564: warning: ‘mem_start’ may be used uninitialized in this function

this seems to be a real bug - depca_eisa_probe() does not check
for failure. Add it, symmetric to depca_isa_probe().

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: David S. Miller <davem@davemloft.net>