bcm2835-sdhost: Workaround for "slow" sectors

Some cards have been seen to cause timeouts after certain sectors are
read. This workaround enforces a minimum delay between the stop after
reading one of those sectors and a subsequent data command.

Using CMD23 (SET_BLOCK_COUNT) avoids this problem, so good cards will
not be penalised by this workaround.

Signed-off-by: Phil Elwell <phil@raspberrypi.org>