spi/mmc_spi: mmc_spi adaptations for SPI bus locking API

Modification of the mmc_spi driver to use the SPI bus locking API.
With this, the mmc_spi driver can be used together with other SPI
devices on the same SPI bus. The exclusive access to the SPI bus is
now managed in the SPI layer. The counting of chip selects in the probe
function is no longer needed.

Signed-off-by: Ernst Schwab <eschwab@online.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Tested-by: Matt Fleming <matt@console-pimps.org>
Tested-by: Antonio Ospite <ospite@studenti.unina.it>