i2c: pnx: Fix bit definitions

The I2C Control Register bits RFDAIE and RFFIE were mixed up. In addition to
this fix, this patch adds the missing bit DRSIE for completeness.

Signed-off-by: Roland Stigge <stigge@antcom.de>
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>