atm: firestream: check the return value of ioremap() in fs_init()

The function ioremap() in fs_init() can fail, so its return value should
be checked.

Reported-by: TOTE Robot <oslab@tsinghua.edu.cn>
Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>