i2c: pxa: don't error out if there's no pinctrl

The bus recovery patch regresses on OLPC XO-1.75 that has no pinctrl in
its DT.

Fixes: 7c9ec2c5 ("i2c: pxa: implement generic i2c bus recovery")'
Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Wolfram Sang <wsa@kernel.org>