V4L/DVB (9511): cx18: Mark CX18_CPU_DE_RELEASE_MDL as a slow API call

cx18: Mark CX18_CPU_DE_RELEASE_MDL as a slow API call.
Give the encoder time to complete the MDL release before destroying the
encoder internal task.  This avoids an encoder lockup on the next digital
capture and error messages about buffers being returned for an inactive
encoder task handle.

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