mxc-mipi-csi2_yav: s_stream: disable only when enabled

Otherwise "v4l2-ctl -d 0 --stream-mmap --stream-to=test.raw --stream-count=1"
would result in s_stream disable called twice, which makes running wrap
around to 255 and confuses the logic.
2 jobs for hi846_updates_20210216 in 68 minutes and 38 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
failed #327356
librem5 allowed to fail
build:aarch64

00:15:08

passed #327357
librem5:arm64
build:deb

01:08:35

 
Name Stage Failure
failed
build:aarch64 Build
  CC [M]  drivers/hid/hid-sensor-hub.o
CC [M] drivers/hid/hid-sensor-custom.o
LD [M] drivers/hid/hid-picolcd.o
LD [M] drivers/hid/hid-uclogic.o
LD [M] drivers/hid/hid-wiimote.o
AR drivers/hid/built-in.a
LD [M] drivers/hid/wacom.o
AR drivers/built-in.a
ERROR: Job failed: exit code 1