gsm_23_003: add GSM23003_MSISDN_{MIN,MAX}_DIGITS

Add the constant, so it can be used in create-subscriber-on-demand
related patches. ITU-T Rec. E.164 6.1 states that maximum international
number length should be 15. I did not find a source for a minimum
length, but I've added the constant and set it to 1 for consistency
(based on the existing osmo_msisdn_str_valid() function).

Related: OS#2542
Change-Id: Idc74f4d94ad44b9fc1b6d43178f5f33d551ebfb1
diff --git a/src/gsm/gsm23003.c b/src/gsm/gsm23003.c
index 2252f70..a97ed07 100644
--- a/src/gsm/gsm23003.c
+++ b/src/gsm/gsm23003.c
@@ -69,7 +69,7 @@
  */
 bool osmo_msisdn_str_valid(const char *msisdn)
 {
-	return is_n_digits(msisdn, 1, 15);
+	return is_n_digits(msisdn, GSM23003_MSISDN_MIN_DIGITS, GSM23003_MSISDN_MAX_DIGITS);
 }
 
 /*! Determine whether the given IMEI is valid according to 3GPP TS 23.003,