em28xx: regression fix: use DRX-K sync firmware requests on em28xx

As em28xx-dvb will always be initialized asynchronously, there's
no need anymore for a separate thread to load the DRX-K firmware.

Fixes a known regression with kernel 3.6 with tda18271 driver
and asynchronous DRX-K firmware load.

Antti tested it with the following hardware:
        Hauppauge WinTV HVR 930C
        MaxMedia UB425-TC
        PCTV QuatroStick nano (520e)

Tested-by: Antti Palosaari <crope@iki.fi>
Cc: stable@kernel.org #	for Kernel 3.6 - please note that driver location has changed
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>