platsupport: s/ZF_LOGF/ZF_LOGE in serial init

This is not a fatal error: it's up to the app to decide that.

libsel4platsupport/src/serial.c

diff --git a/libsel4platsupport/src/serial.c b/libsel4platsupport/src/serial.c
index 6f4a760..1904840 100644
--- a/libsel4platsupport/src/serial.c
+++ b/libsel4platsupport/src/serial.c
@@ -88,7 +88,7 @@
     /* Allocate slot for the PS default serial's IRQ cap. */
     error = vka_cspace_alloc_path(vka, &serial_objects->serial_irq_path);
     if (error) {
-        ZF_LOGF("Failed to allocate serial IRQ slot.");
+        ZF_LOGE("Failed to allocate serial IRQ slot.");
         return error;
     }
 
@@ -99,7 +99,7 @@
      */
     error = sel4platsupport_arch_init_default_serial_caps(vka, vspace, simple, serial_objects);
     if (error) {
-        ZF_LOGF("Arch-specific serial cap init failed.");
+        ZF_LOGE("Arch-specific serial cap init failed.");
         return error;
     }