i2c: sh_mobile: remove get_data function

It makes the code much easier comprehensible to explicitly code that the
first byte will be client address and all the following bytes are the
actual data.

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>