V4L/DVB (6540): em28xx: fix failing autodetection after the reboot

The attached patch is required so that the autodetecion code also works after
a reboot.

Setting the I2C speed does not seem to be supported for em2800.

Signed-off-by: Sascha Sommer <saschasommer@freenet.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>