mctp i2c: drop check because i2c_unregister_device() is NULL safe

No need to check the argument of i2c_unregister_device() because the
function itself does it.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://patch.msgid.link/20241202082713.9719-1-wsa+renesas@sang-engineering.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>