Allow SD card SPI speed to be selected in settings

[and3rson]

Some cards support speed up to 50 MHz.

I suggest the following choices:

• 4 MHz
• 10 MHz
• 20 MHz (current default)
• 25 MHz (maximum allowed by SD lib)

Default is 4 MHz. Our default (in sdk) is 20 MHz. My class 10 card works even @ 25 MHz without issues.

[frostmorn]

I've tried worst ever card I ever had, and it supports 20 MHz without problems.
We could try to initialize card at max frequency, and if card is recognized okay(cardType != CARD_UNKNOWN || cardType != CARD_NONE) we could proceed, otherwise
drop to lower values and retry, without notifying user at all.

[frostmorn]

It seems that 25 Mhz chosen here as a Default speed value any SD card should support

Some cards support even much more, so init card at 25 Mhz should be okay
But to be sure, we could adjust frequency as I mentioned before