isdn: return -EFAULT if copy_from_user() fails

We should be returning -EFAULT here.

Mostly this patch is to silence a smatch warning.  The upper levels
of this driver turn all non-zero return values from isar_load_firmware()
into 1.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>