eicon: Fixed checkpatch warning

drivers/isdn/hardware/eicon/diva_didd.c:32:6: warning: symbol
'DRIVERRELEASE_DIDD' was not declared. Should it be static?

Signed-off-by: Andreea Hodea <hodea_andreea@yahoo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>