V4L/DVB (7540): em28xx: convert to use videobuf-vmalloc

The usage of videobuf-vmalloc allows to cleanup em28xx logic.

Also, it reduced its size by about 5.42% on i386 arch (and about 7.5% on x86_64):

  39113    4876      40   44029    abfd old/em28xx.ko
  36731    4868      40   41639    a2a7 /home/v4l/master/v4l/em28xx.ko

Also, the preliminary tests, made on a single core 1.5 MHz Centrino showed
that CPU usage reduced from 42%-75% to 28%-33% (reports from "top") command.

A test with time command presented an even better result:

This is the performance tests I did, running code_example to get 1,000 frames
@29.995 Hz (about 35 seconds of stream), tested on a i386 machine, running at
1,5GHz:

	The old driver:

$ time -f "%E: %Us User time, %Ss Kernel time, %P CPU used" ./capture_example
0:34.21: 8.22s User time, 25.16s Kernel time, 97% CPU used

	The videobuf-based driver:

$ time -f "%E: %Us User time, %Ss Kernel time, %P CPU used" ./capture_example
0:35.36: 0.01s User time, 0.05s Kernel time, 0% CPU used

	Conclusion:

The time consumption to receive the stream where reduced from about 33.38
seconds to 0.05 seconds.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>