USB Serial Sierra: clean-up

Very minor changes to clean up sierra code. Adds a prefix to debug messages so
that Sierra messages are easily recognized. Removes extraneous code.

This targets kernel 2.6.26-rc9

Signed-off-by: Kevin Lloyd <klloyd@sierrawireless.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>