atm: fix improper return value

It returns variable "error" when ioremap_nocache() returns a NULL
pointer. The value of "error" is 0 then, which will mislead the callers
to believe that there is no error. This patch fixes the bug, returning
"-ENOMEM".

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=189021

Signed-off-by: Pan Bian <bianpan2016@163.com>
Signed-off-by: David S. Miller <davem@davemloft.net>