misc: Doxygen tweaks: fixed some typos and minor errors

Doxygen generates quite a lot of warnings on libosmocore. Some of them
are obvious typos - this patch aims to fix such low-hanging fruit.
diff --git a/src/serial.c b/src/serial.c
index a025ae9..66ee756 100644
--- a/src/serial.c
+++ b/src/serial.c
@@ -27,7 +27,7 @@
  */
 
 /*! \file serial.c
- *  \file Osmocom serial port helpers
+ * Osmocom serial port helpers
  */
 
 #include <errno.h>