net: hinic: Use devm_kasprintf()

Use devm_kasprintf() instead of hand writing it.
This is less verbose and less error prone.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>