USB: cdc-acm: cleanup of data structures

Buffers should be u8*, not unsigned char*
Buffers have an unsigned length and using an int
as a boolean is a bit outdated.

No functional change intended.

Signed-off-by: Oliver Neukum <oneukum@suse.com>
Link: https://lore.kernel.org/r/20200917110235.11854-1-oneukum@suse.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>