Support controllers with lower block size limits
Some controllers (such as the eMMC controller in the Pinebook) require transfers to be limited to a smaller number of blocks per operation than is permitted by a generic SDHCI controller.
Requires RiscOS/Sources/HWSupport/SD/SDIODriver!6 (merged) in order to build.
Requires Support/CI_Source!24 (merged) in order to pass CI.