fore200: fix oops on failed firmware load

Fore 200 ATM driver fails to handle request_firmware failures and oopses 
when no firmware file was found. Fix it by checking for the right return 
values and propaganting the return value up.

Signed-off-by: Meelis Roos <mroos@linux.ee>
Signed-off-by: David S. Miller <davem@davemloft.net>