USB: cdc-wdm: fix regression on buffer deallocation

USB: cdc-wdm: fix regression on buffer deallocation

[ no commit upstream, since usb_alloc_coherent usage was removed from
  the driver before this was fixed ]

Commit cafbe85 ("USB: cdc-wdm: better allocate a buffer that is at least
as big as we tell the USB core") introduced a regression: it changed
the size used in usb_alloc_coherent, but failed to do the same for the
usb_free_coherent calls.

It also was marked for stable and got backported to older kernels. But
in the upstream kernel, the usage of usb_alloc_coherent was right after
removed from the driver, so upstream doesn't have this problem, while
the stable kernels still have, and thus need this fix.

BugLink: https://bugs.launchpad.net/bugs/1074157
Signed-off-by: Herton Ronaldo Krzesinski <herton.krzesinski@canonical.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>