Remove protected word "Matrix"

I am not sure how you've been getting this to compile, but I think the word "Matrix" was accidentally added to the struct definition when you made your changes to the Adafruit library initially. I was unable to compile without removing the word.

```
C:\.../RA8875.h:236:16: error: using typedef-name 'Matrix' after 'struct'
 typedef struct Matrix { int32_t An,Bn,Cn,Dn,En,Fn,Divider ; } tsMatrix_t;
                ^
In file included from C:\Program Files (x86)\Arduino 1.5.8b\hardware\arduino\sam\system/CMSIS/Device/ATMEL/sam3xa/include/sam3x8e.h:282:0,
                 from C:\Program Files (x86)\Arduino 1.5.8b\hardware\arduino\sam\system/CMSIS/Device/ATMEL/sam3xa/include/sam3xa.h:44,
                 from C:\Program Files (x86)\Arduino 1.5.8b\hardware\arduino\sam\system/CMSIS/Device/ATMEL/sam3.h:59,
                 from C:\Program Files (x86)\Arduino 1.5.8b\hardware\arduino\sam\system/CMSIS/Device/ATMEL/sam.h:198,
                 from C:\Program Files (x86)\Arduino 1.5.8b\hardware\arduino\sam\system/libsam/chip.h:25,
                 from C:\Program Files (x86)\Arduino 1.5.8b\hardware\arduino\sam\cores\arduino/Arduino.h:42,
                 from C:\Program Files (x86)\Arduino 1.5.8b\hardware\arduino\sam\variants\arduino_due_x/variant.h:36,
                 from C:\Program Files (x86)\Arduino 1.5.8b\hardware\arduino\sam\libraries\SPI/SPI.h:15,
                 from clock.ino:9:
C:\Program Files (x86)\Arduino 1.5.8b\hardware\arduino\sam\system/CMSIS/Device/ATMEL/sam3xa/include/component/component_matrix.h:71:3: note: 'Matrix' has a previous declaration here
 } Matrix;
   ^
C:\.../RA8875.h:236:73: error: invalid type in declaration before ';' token
 typedef struct Matrix { int32_t An,Bn,Cn,Dn,En,Fn,Divider ; } tsMatrix_t;
                                                                         ^
Error compiling.
```