Avoid warning about missing return

[SimonFJ20]

When compiling a sketch with Mbed OPTA board selected, you get the following warning, if you have enabled All compiler warnings.

/home/simon/.arduino15/packages/arduino/hardware/mbed_opta/4.0.10/variants/OPTA/variant.cpp: In member function 'virtual int SecureQSPIFBlockDevice::programSecure(void*, mbed::bd_addr_t, mbed::bd_size_t)':
/home/simon/.arduino15/packages/arduino/hardware/mbed_opta/4.0.10/variants/OPTA/variant.cpp:266:5: warning: no return statement in function returning non-void [-Wreturn-type]
     }
     ^
/home/simon/.arduino15/packages/arduino/hardware/mbed_opta/4.0.10/variants/OPTA/variant.cpp: In member function 'virtual int SecureQSPIFBlockDevice::eraseSecure(void*, mbed::bd_addr_t, mbed::bd_size_t)':
/home/simon/.arduino15/packages/arduino/hardware/mbed_opta/4.0.10/variants/OPTA/variant.cpp:269:5: warning: no return statement in function returning non-void [-Wreturn-type]
     }
     ^

To remove this warning there are 2 options:

1. Return some value
2. Use GCC #pragma directive to turn off warnings temporarily

I chose the first option, but a comment right above the class says // 256byte secure OTP area (on AT25SF128A), and I'm not sure if this means that the class/function definitions will produce too large outputs (>256). In which case, option 2 should be used instead IMO.

[CLAassistant]

All committers have signed the CLA.

[facchinm]

Merged, thank you!