chan_simpleusb/chan_usbradio: Add function document and code clean up

chan_simpleusb and chan_usbradio functions have been fully documented.

A diff was performed on the two drivers and the functions changed to match as close as possible. Some routines were moved so that the functions are in the same order. (This prepares the code for more refactoring with res_usbradio.)

Routines that directly modify audio data were updated to use the register modifier on loop variables. This serves as a hint to the compiler. Since we compile without optimization, this can help performance. This matches the use of register in asterisk codecs.

Unneeded includes were removed. Unneeded private structure variables were removed (there were a number removed). Code that did nothing was removed. Some variable names were changed to better match their intended use.

Addressed a todo about using &ast_null_frame instead of creating a null frame.

Configuration variables that only held true or false values where changed to a single bit. Those were grouped together so that the compiler could pack them into a single integer - which saves memory space.

Added the txoffdelay feature to chan_simpleusb. This was already present in chan_usbradio. The feature set closely matches. chan_usbradio still needs to support pocsag paging.

radio tune and radio tune commands will now show the sub commands when tab is pressed after entering the base command.

AllStarLink / app_rpt

chan_simpleusb/chan_usbradio: Add function document and code clean up #244