libmm-glib: fix enum conversion warning/error

mm-modem-oma.c:400:51: error: implicit conversion from enumeration type 'MMOmaSessionType' to different enumeration type 'MMOmaFeature'
      [-Werror,-Wenum-conversion]
    g_return_val_if_fail (MM_IS_MODEM_OMA (self), MM_OMA_SESSION_TYPE_UNKNOWN);
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/build/oak/usr/include/glib-2.0/glib/gmessages.h:382:11: note: expanded from macro 'g_return_val_if_fail'
         return (val);                                                  \
         ~~~~~~  ^~~

This should be MMOmaFeature, not MMOmaSessionType. The end result is the same,
as both are 0.

BUG=chromium:597446
TEST=`emerge-oak modemmanager-next` (with and without upgraded glib)

Change-Id: I9d46bd9d5ee34cfb0b2ccece4d7526179110e9ec
Signed-off-by: Brian Norris <briannorris@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/334730
Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>
diff --git a/libmm-glib/mm-modem-oma.c b/libmm-glib/mm-modem-oma.c
index 380bb08..77c682d 100644
--- a/libmm-glib/mm-modem-oma.c
+++ b/libmm-glib/mm-modem-oma.c
@@ -397,7 +397,7 @@
 MMOmaFeature
 mm_modem_oma_get_features  (MMModemOma *self)
 {
-    g_return_val_if_fail (MM_IS_MODEM_OMA (self), MM_OMA_SESSION_TYPE_UNKNOWN);
+    g_return_val_if_fail (MM_IS_MODEM_OMA (self), MM_OMA_FEATURE_NONE);
 
     return (MMOmaFeature) mm_gdbus_modem_oma_get_features (MM_GDBUS_MODEM_OMA (self));
 }