i2c-algo-pca: Fix use of uninitialized variable in debug message

A recent change broke debugging of pca_xfer(), fix it.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Wolfram Sang <w.sang@pengutronix.de>