V4L/DVB (7188): radio-si470x version 1.0.6

This patch combines all the finished discussions and its resulting patches from
the mailing list.

The version 1.0.6 is mainly influenced by Oliver Neukum. He found a lot of
small issues, that are fixed with this patch now. For me the most interesting
thing is, that it's now safer to use it on other architectures.

The history for version 1.0.6 is:
- fixed coverity checker warnings in *_usb_driver_disconnect
- probe()/open() race by correct ordering in probe()
- DMA coherency rules by separate allocation of all buffers
- use of endianness macros
- abuse of spinlock, replaced by mutex
- racy handling of timer in disconnect, replaced by delayed_work
- racy interruptible_sleep_on(), replaced with wait_event_interruptible()
- handle signals in read()

The driver is tested with all Debian/testing radio programs and rdsd. The patch
is tested against checkpatch.pl v1.12.
Signed-off-by: Tobias Lorenz <tobias.lorenz@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>