atm: lanai: set error code when ioremap fails

In function lanai_dev_open(), when the call to ioremap() fails, the
value of return variable result is 0. 0 means no error in this context.
This patch fixes the bug, assigning "-ENOMEM" to result when ioremap()
returns a NULL pointer.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=188791

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