msc/sccp_ran.h: fix: do not pass -1 to osmo_rat_type_name()

The problem is that osmo_rat_type_name() calls get_value_string(),
so we first cast -1 to 'const enum osmo_rat_type' and then to
'uint32_t'. Let's rather use OSMO_RAT_UNKNOWN.

Found by GCC with -Wextra in CFLAGS:

  warning: operand of ?: changes signedness from ‘int’ to
	   ‘const enum osmo_rat_type’ due to unsignedness
	   of other operand [-Wsign-compare]

Change-Id: I63ba355102d3cc035ba90121e06aba7cf1776aa0
1 file changed