usb: gadget: f_acm: Fix configfs attr name

Correct attribute name is port_num not num.

Fixes: ea6bd6b1 ("usb-gadget/f_acm: use per-attribute show and store methods")
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>