i2c: core: remove level of indentation in i2c_transfer

Using the common kernel pattern to bail out at the beginning if some
conditions are not met, we can save a level of indentation. No
functional change.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>