i2c: dev: don't start function name with 'return'

I stumbled multiple times over 'return_i2c_dev', especially before the
actual 'return res'. It makes the code hard to read, so reanme the
function to 'put_i2c_dev' which also better matches 'get_free_i2c_dev'.

Signed-off-by: Wolfram Sang <wsa@the-dreams.de>