[ARM] 4827/1: fix two warnings in drivers/i2c/busses/i2c-pxa.c

This fixes two warnings:

* unused static defined function decode_ICR() when
  compiled without CONFIG_I2C_PXA_SLAVE
* a sparse warning about a void function returning
  something

Signed-off-by: Holger Schurig <hs4233@mail.mn-solutions.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>