Revert "Return error code"

This reverts commit 0c466fe194259ff10a58ae1c1d8beb895d034bcb.

Do it properly...
handle memory allocation failure

_rtlsdr_alloc_async_buffers() can fail, handle gracefully.
add r82xx_enable_manual_gain()

Split setting gain from enabling manual gain - as for the other
tuner implementations.
e4k: interface to set individual stage gains
e4k: fix small typo in table

IF gain for stage 4 is wrong: fix according to data sheet
r82xx: interface to set individual stage gains
r82xx: code looks wrong?!?

the way it is implemented, data will overlap
extend librtlsdr API: set tuner stage gains

add rtlsdr_get_tuner_stage_gains() to get a description of the
internal hardware tuner stages and their gain settings.

rtlsdr_set_tuner_stage_gain() will set the gain of a certain stage.

TODO: dev->gain is not yet updated
add some test code for rtlsdr_get_tuner_stage_gains()