doxygen: enable AUTOBRIEF, drop \brief

Especially for short descriptions, it is annoying to have to type \brief for
every single API doc.

Drop all \brief and enable the AUTOBRIEF feature of doxygen, which always takes
the first sentence of an API doc as the brief description.

Change-Id: I11a8a821b065a128108641a2a63fb5a2b1916e87
diff --git a/src/gsm/gprs_cipher_core.c b/src/gsm/gprs_cipher_core.c
index a0cf185..b833048 100644
--- a/src/gsm/gprs_cipher_core.c
+++ b/src/gsm/gprs_cipher_core.c
@@ -88,7 +88,7 @@
 	return selected_ciphers[algo]->run(out, len, kc, iv, dir);
 }
 
-/*! \brief Obtain key lenght for given GPRS cipher
+/*! Obtain key lenght for given GPRS cipher
  *  \param[in] algo Enum representive GPRS cipher
  *  \returns unsigned integer key length for supported algorithms,
  *  for GEA0 and unknown ciphers will return 0