gigaset: include cleanup cleanup

Commit 5a0e3ad6 causes slab.h to be included twice in many of the
Gigaset driver's source files, first via the common include file
gigaset.h and then a second time directly. Drop the spares, and
use the opportunity to clean up a few more similar cases.

Impact: cleanup, no functional change
Signed-off-by: Tilman Schmidt <tilman@imap.cc>
CC: Tejun Heo <tj@kernel.org>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>