SPI SD-Card: setup valid CRC-7 for STOP_TRANSMISSION command.

STOP_TRANSMISSION command is used to finish READ_MULTIPLE_BLOCK
command and its format is regular command format.
It requires valid CRC-7 to have effect at least on
same cards types else it is ignored and attempt
to issue next READ or WRITE commands results in
illegal command condition (0x04) preceded by strange
(0x3f) for tested card.

Signed-off-by: Pavel Pisa <ppisa@pikron.com>