doxygen: fix many warnings on "end of file while in group"

lib/stm32/common/flash_common_all.c

@@ -49,4 +49,4 @@ void flash_unlock_option_bytes(void)
 	FLASH_OPTKEYR = FLASH_OPTKEYR_KEY2;
 }
 
-/*@}*/
+/**@}*/

lib/stm32/common/opamp_common_all.c

@@ -109,4 +109,4 @@ void opamp_vm_select(uint32_t base, uint32_t vm)
 	OPAMP_CSR(base) |= vm << OPAMP_CSR_VM_SEL_SHIFT;
 }
 
-/*@}*/
+/**@}*/

lib/stm32/common/opamp_common_v1.c

@@ -53,4 +53,4 @@ void opamp_vms_select(uint32_t base, uint32_t vms)
 	OPAMP_CSR(base) |= vms << OPAMP_CSR_VMS_SEL_SHIFT;
 }
 
-/*@}*/
+/**@}*/

lib/stm32/common/opamp_common_v2.c

@@ -49,4 +49,4 @@ void opamp_output_set_external(uint32_t base)
 	OPAMP_CSR(base) &= ~OPAMP_CSR_OPAINTOEN;
 }
 
-/*@}*/
+/**@}*/

lib/stm32/g4/adc.c

@@ -27,7 +27,6 @@
  * You should have received a copy of the GNU Lesser General Public License
  * along with this library.  If not, see <http://www.gnu.org/licenses/>.
  */
-/**@{*/
 
 #include <libopencm3/stm32/adc.h>