If strdup() fails with out-of-memory, __dev_add_param() should fail
with -ENOMEM.

(Note strdup() is always given with a non-NULL pointer.  If the
argument name is given with NULL, the system would already have
crashed in the get_param_by_name() function.)

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>