V4L/DVB (13426): cx18: Rename struct cx18_mdl to struct cx18_mdl_ent

Rename type in anticipation of implementing a struct cx18_mdl type that
actually keeps track of a memory descriptor list.

Signed-off-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>