spi: soft_spi: fix miso gpio property name

The patch fix a missprint introduced in commit 2e9fe73a ("spi: soft_spi:
Support the recommended soft spi properties").

Signed-off-by: Mikhail Kshevetskiy <mikhail.kshevetskiy@iopsys.eu>
Reviewed-by: Fabio Estevam <festevam@gmail.com>