spi_tranfer: factorize common code to simplify maintenance

[ABOSTM]

Summary
Hi @stas2z
I review the PR
stm32duino#912
That is nice job and looks good to me.
Nevertheless I propose to factorize common code to improve readability and maintenance.
I does not impact performances. (tested on NUCLEO_L476RG with LCD/SD sadafruit shield).

[stas2z]

Actually, im sure it does impact perfomance cuz it have to check skipReceive at every byte sent instead of checking it just before transfer loop start

ive checked it with my lcds, it takes about 5% of transfer perfomance (simple buffer-to-screen test case) no matter of optimisations enabled (ive checked -Os/-O2/-O3 as most common)

sure it's not so much and probably we can waste it for better readability

[ABOSTM]

I am surprised, because:

• My LCD library call transfer() for each byte, so it doesn't make a difference
• If library send multiple bytes at once, then you are right, the check is done at each byte, but it is done while byte is being transferred so it should not be visible.

Whatever, as you said 5% is worth for better readability.
So if you agree, you can merge this PR (as it is in your fork) and it will automatically push the update to Arduino_Core_STM32 into your original PR