Declare spiTimerTickStart and spiTimerTickDelay as static

[vpreatoni]

Hi!
great work!! I've seen very crappy code regarding SD SPI and STM32 but yours is very clean and tidy.

Just a suggestion, declare instead:

static uint32_t spiTimerTickStart;
static uint32_t spiTimerTickDelay;

They are not used outside user_diskio_spi.c file, so it is a good practice declare them static globals.

[kiwih]

Hi there, you can submit a PR with this if you would like, I will accept it.