[media] em28xx: Do not modify EM28XX_R06_I2C_CLK for em2800

writing the EM28XX_R06_I2C_CLK register leads to the problem that the
i2c bus on the Terratec Cinergy 200 USB is no longer usable when the
system is rebooted.

The device needs to be unplugged in order to bring it back to life.
Attached patch conditionally disables the write in
em28xx_pre_card_setup() like it is already done in em28xx_card_setup().

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