USB: serial mct_usb232: move DMA buffers to heap

My distro kernel (Fedora Rawhide) started throwing warnings from DMA API
checker, so I have no choice but band-aid it quick. There's no attempt
to reuse DMA buffers. Control messages are only sent rarely anyway.

Signed-off-by: Pete Zaitcev <zaitcev@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>