em28xx: Make all em28xx extensions to be initialized asynchronously

em28xx-dvb, em28xx-alsa and em28xx-ir are typically initialized
asyncrhronously. The exception for it is when those modules
are loaded before em28xx (or before an em28xx card insertion) or
when they're built in.

Make the extentions to always load asynchronously. That allows
having all DVB firmwares loaded synchronously with udev-182.

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>