i2c: sh_mobile: fix timeout error handling

In a timeout case return an error immediately from the driver's
.master_xfer() method, instead of continuing and letting higher layers
fail.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Wolfram Sang <wolfram@the-dreams.de>