i2c: sh_mobile: remove is_first_byte function

All state machines deal with pd->pos values. This helper function is an
exception and makes it only more confusing.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>