V4L/DVB (7091): radio-si470x improvements and seldom problem fixed in tuning functions

I updated the radio-si470x driver another time. Here are the commented history entries:

- number of seek_retries changed to tune_timeout
The last versions checked for the end of frequency tuning by polling a si470x register.
Therefore polling depended on the usb utilization.
This was changed to have a constant timeout now.

- fixed problem with incomplete tune operations by own buffers
The last version used a shared buffer to assembly the USB HID reports.
It sometimes happened, that multiple functions were modifing this buffer simultanuously.
When sending such reports, the hardware returned USB stalls (-EPIPE).
Now buffers of the correct size (smaller than before) are allocated as local variables.

- optimization of variables
The size of some variables has been reduced to allow the compiler to generate more optimized code.

- improved error logging
At some important location, error checking was improved.
Especially the usb transfers to access si470x registers and the tuning functions were modified.

Signed-off-by: Tobias Lorenz <tobias.lorenz@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>