hw/wm8750: Fix potential buffer overflow

Report from smatch:

hw/wm8750.c:369 wm8750_tx(12) error: buffer overflow 's->i2c_data' 2 <= 2

It looks like the preprocessor statements were simply misplaced.

Replace also __FUNCTION__ by __func__ to please checkpatch.pl.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org> 
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
