V4L/DVB (4970): Usbvision memory fixes

- fix decompression buffer allocation not done at first driver open
- simplification of USB sbuf allocation (use of usb_buffer_alloc)
- replaced vmalloc by vmalloc_32 (for homogeneity)
- add of saa7111 (i2cAddr=0x48) detection printout in attach_inform

Signed-off-by: Thierry MERLE <thierry.merle@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>