Add support for multiple write block transfers to SD

[ostenning]

• Add support for multiple write block transfers to SD
• As the sd_cmd::set_block_count is specific to SD cards, this function is available for SD and not other mediums.

https://github.com/stm32-rs/stm32h7xx-hal/issues/454

cc @richardeoin

[ostenning]

Closed in favor of https://github.com/stm32-rs/stm32h7xx-hal/pull/453