gcc 3.3.3 gave the following error when compiling sis190.c

drivers/net/sis190.c: In function 'sis190_get_mac_addr_from_apc':
drivers/net/sis190.c:966: warning: 'isa_bridge' might be used
uninitialized in this function
make: *** [bin/sis190.o] Error 1

This patch allows error-free compilation.

Signed-off-by: Marty Connor <mdc@etherboot.org>