[ATM]: Fix warning.

The compiler warning

drivers/atm/firestream.c: In function ‘top_off_fp’:
drivers/atm/firestream.c:1505: warning: cast to pointer from integer of different size

does indicate a bug, albeit a minor one.  Fixed, by using a 32-bit
temporary prior to the call to bus_to_virt().

The larger bug is still present:  the entire driver assumes that machine
pointers are 32-bit, as it stores pointers in 32-bit hardware registers.
This is obvious to anyone who knows the driver well, but for the casual
readers it is helpfully noted with FIXME.

Signed-off-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: David S. Miller <davem@davemloft.net>