usb: gadget: f_acm: Fix configfs attr name

Correct attribute name is port_num not num.

Fixes: ea6bd6b ("usb-gadget/f_acm: use per-attribute show and store methods")
Reviewed-by: Christoph Hellwig <[e-mail]>
Signed-off-by: Krzysztof Opasiak <[e-mail]>
Signed-off-by: Felipe Balbi <[e-mail]>

drivers/usb/gadget/function/f_acm.c

···
 	return sprintf(page, "%u\n", to_f_serial_opts(item)->port_num);
 }
 
-CONFIGFS_ATTR_RO(f_acm_port_, num);
+CONFIGFS_ATTR_RO(f_acm_, port_num);
 
 static struct configfs_attribute *acm_attrs[] = {
-	&f_acm_port_attr_num,
+	&f_acm_attr_port_num,
 	NULL,
 };