[ATM] firestream: handle thrown error

gcc emits the following warning:

drivers/atm/firestream.c: In function ‘fs_open’:
drivers/atm/firestream.c:870: warning: ‘tmc0’ may be used uninitialized in this function

This indicates a real bug.  We should check make_rate() return value for
potential errors.

Signed-off-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: Adrian Bunk <bunk@stusta.de>