cmd: i2c: Fix help output of i2c command.

In case SYS_I2C or DM_I2C are defined, then the "i2c " prefix
of the "i2c crc32" command is missing.
This patch addresses this, so that users can't get confused
by the "crc32" command.

Without the patch we get

    => i2c help
    i2c - I2C sub-system

    Usage:
    i2c bus [muxtype:muxaddr:muxchannel] - show I2C bus info
    crc32 chip address[.0, .1, .2] count - compute CRC32 checksum
    i2c dev [dev] - show or set current I2C bus
    [...]

With the patch we get

    => i2c help
    i2c - I2C sub-system

    Usage:
    i2c bus [muxtype:muxaddr:muxchannel] - show I2C bus info
    i2c crc32 chip address[.0, .1, .2] count - compute CRC32 checksum
    i2c dev [dev] - show or set current I2C bus
    ...

Signed-off-by: Christoph Muellner <christoph.muellner@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Heiko Schocher <hs@denx.de>