V4L/DVB (7179): Allow more than one em28xx board

em28xx driver is capable of handling more than one usb device. However, isoc
transfers require a large amount of data to be transfered.

Before this patch, just one em28xx board were enough to allocate more than 50%
URBs:

T:  Bus=02 Lev=00 Prnt=00 Port=00 Cnt=00 Dev#=  1 Spd=480 MxCh= 8
B:  Alloc=480/800 us (60%), #Int=  0, #Iso=  2
D:  Ver= 2.00 Cls=09(hub  ) Sub=00 Prot=01 MxPS=64 #Cfgs=  1

So, only one board could use an USB host at the same time. After the patch, it
is possible to use more than one em28xx at the same time, on the same usb host,
if the image size is slower or equal to 345600, since those images will
require about 30% of the URBs:

T:  Bus=02 Lev=00 Prnt=00 Port=00 Cnt=00 Dev#=  1 Spd=480 MxCh= 8
B:  Alloc=232/800 us (29%), #Int=  0, #Iso=  2
D:  Ver= 2.00 Cls=09(hub  ) Sub=00 Prot=01 MxPS=64 #Cfgs=  1

So, in thesis, after the patch, it would be possible to use up to 3 boards by
each usb host, if the devices are generating small images.

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