i2c: soft: Fix typo in CONFIG_SYS_I2C_SOFT_SPEED

In case only the CONFIG_SYS_I2C_SPEED is set in configuration file,
the CONFIG_SYS_I2C_SOFT_SPEED is defined as CONFIG_SYS_I2C_SPEED.
The CONFIG_SYS_I2C_SOFT_SPEED is then used throughout the driver.

Unfortunatelly, due to a typo in the driver, instead of defining
CONFIG_SYS_I2C_SOFT_SPEED, an CONFIG_SYS_SOFT_I2C_SPEED was defined
and therefore the driver failed to compile. The same applies for
CONFIG_SYS_I2C_SOFT_SLAVE , where the swap happens as well.

This patch fixes the issue.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Heiko Schocher <hs@denx.de>