usb_gigaset: code cleanup

Reorganize the code of the Gigaset M10x driver to make it more
readable, less redundant, better aligned to the style of other
parts of the driver, and cause fewer checkpatch.pl complaints.

Impact: code reorganization, no functional change
Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>