Fix "call to overloaded transfer() is ambiguous" issue

[board707]

Two overloaded SPI.transfer()methods in file STM32F1/libraries/SPI/src/SPI.h:

void transfer(const void * buffer, size_t length);

void transfer(uint8_t * trx_buf, uint32 len);

produces a "call is ambiguous" error if called as:
spi.transfer(uint8*, size_t);

Making the type of second parameter the same in both methods eliminates the error.

[stevstrong]

Thank you.