USB: serial: keyspan: drop header file

Move all declarations and definitions in keyspan.h to keyspan.c, which
is the only place were they are used.

This specifically moves the driver device-id tables and usb-serial
driver definitions to the source file where they are expected to be
found.

While at it, fix up some multi-line comments and minor white-space
issues (spaces instead of tabs and superfluous white space).

Note that the information in the comment header of the removed header
file is also present in the source file.
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>