media: em28xx: Fix misplaced reset of dev->v4l::field_count

The backport of commit afeaade90db4 "media: em28xx: make
v4l2-compliance happier by starting sequence on zero" added a
reset on em28xx_v4l2::field_count to em28xx_ctrl_notify(),
but it should be done in em28xx_start_analog_streaming().

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>