Fix F103T compilation error: remove const

Method adc_default_config still had the adc_dev* 'const' for F103T, but this doesn't compile.

For other boards, the 'const' was removed already.  See for example commits 0b34af3 and
2cdbbc8.

Tested on Arduino IDE 1.6.13.

Compilation error message was along these lines:
STM32F1\variants\generic_stm32f103t\wirish\boards.cpp:183:34: error: invalid conversion from 'const adc_dev*' to 'adc_dev*' [-fpermissive]
STM32F1\system/libmaple/include/libmaple/adc.h:306:13: error:   initializing argument 1 of 'void adc_enable_single_swstart(adc_dev*)' [-fpermissive]
STM32F1\variants\generic_stm32f103t\wirish\boards.cpp:184:53: error: invalid conversion from 'const adc_dev*' to 'adc_dev*' [-fpermissive]
STM32F1\system/libmaple/include/libmaple/adc.h:268:6: error:   initializing argument 1 of 'void adc_set_sample_rate(adc_dev*, adc_smp_rate)' [-fpermissive]
STM32F1\variants\generic_stm32f103t\wirish\boards.cpp: In function 'void setup_adcs()':
STM32F1\variants\generic_stm32f103t\wirish\boards.cpp:189:35: error: invalid conversion from 'void (*)(const adc_dev*)' to 'void (*)(adc_dev*)' [-fpermissive]
STM32F1\system/libmaple/include/libmaple/adc.h:282:13: error:   initializing argument 1 of 'void adc_foreach(void (*)(adc_dev*))' [-fpermissive]

Author: florisla

STM32F1/variants/generic_stm32f103t/wirish/boards.cpp

@@ -179,7 +179,7 @@ nvic_init((uint32)VECT_TAB_ADDR, 0);
 */
 }
 
-static void adc_default_config(const adc_dev *dev) {
+static void adc_default_config(adc_dev *dev) {
     adc_enable_single_swstart(dev);
     adc_set_sample_rate(dev, wirish::priv::w_adc_smp);
 }