isdn/gigaset: beautify ev-layer.c

Cosmetic changes to drivers/isdn/gigaset/ev-layer.c and
drivers/isdn/gigaset/gigaset.h to improve readability.

Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>