modem-helpers: improve +CMTI indication regex

Allow whitespaces added in several places, like between the comma and the index,
which is what the Cinterion PHS8 does:
  <CR><LF>+CMTI: "MT", 5<CR><LF>

src/mm-modem-helpers.c

@@ -495,7 +495,7 @@ mm_3gpp_cusd_regex_get (void)
 GRegex *
 mm_3gpp_cmti_regex_get (void)
 {
-    return g_regex_new ("\\r\\n\\+CMTI: \"(\\S+)\",(\\d+)\\r\\n",
+    return g_regex_new ("\\r\\n\\+CMTI:\\s*\"(\\S+)\",\\s*(\\d+)\\r\\n",
                         G_REGEX_RAW | G_REGEX_OPTIMIZE,
                         0,
                         NULL);