i2c-pxa: fix compiler warning, due to missing const

This patch adds the missing const to "struct platform_device_id" to fix
this warning:

/home/frogger/pengutronix/linux/linux-2.6/drivers/i2c/busses/i2c-pxa.c:
In function 'i2c_pxa_probe':
/home/frogger/pengutronix/linux/linux-2.6/drivers/i2c/busses/i2c-pxa.c:1004:
warning: initialization discards qualifiers from pointer target type

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Cc: Eric Miao <eric.y.miao@gmail.com>
Cc: Roel Kluin <roel.kluin@gmail.com>
Cc: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>