gigaset: silence GCC warning for unused 'format_ie'

Building Gigaset's CAPI support without Gigaset's debugging enabled
triggers this GCC warning:
    'format_ie' defined but not used [-Wunused-function]

Silence this warning by wrapping format_ie() in an "#ifdef
CONFIG_GIGASET_DEBUG" and "#endif" pair.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: David S. Miller <davem@davemloft.net>