Fixes compile errors for massdrop keyboards (#70)

changelog.md

@@ -28,5 +28,6 @@
 05-29-2019 - Fixing matrix_scan so it properly returns changed status  
 05-29-2019 - Add belgian layour for sendstring (qmk#6008)  
 06-03-2019 - Overhaul of AutoShift feature (qmk#6067)  
+06-05-2019 - Fix compiler error for ATSAM based keyboards  
 06-05-2019 - Move RGB typedef's into a unified location   
 06-05-2019 - upgrade gcc-arm-none-eabi from the default 5.4.1 to 6.3.1 due to ARM runtine issues   

tmk_core/protocol/arm_atsam/md_bootloader.h

@@ -11,7 +11,7 @@ extern uint32_t _erom;
 //WARNING: These are only for CTRL bootloader release "v2.18Jun 22 2018 17:28:08" for bootloader_jump support
 extern uint32_t _eram;
 #define BOOTLOADER_MAGIC 0x3B9ACA00
-#define MAGIC_ADDR (uint32_t *)(&_eram - 4)
+#define MAGIC_ADDR (uint32_t *)((intptr_t)(&_eram) - 4)
 #endif
 
 #ifdef MD_BOOTLOADER
@@ -22,4 +22,3 @@ extern uint32_t _eram;
 #endif //MD_BOOTLOADER
 
 #endif //_MD_BOOTLOADER_H_
-