i2c: altera: Fix race between xfer_msg and isr thread

Use a mutex to protect access to idev->msg_len, idev->buf, etc. which
are modified by both altr_i2c_xfer_msg() and altr_i2c_isr().

This is the minimal fix for easy backporting. A cleanup to remove the
spinlock will be added later.

Signed-off-by: Atsushi Nemoto <atsushi.nemoto@sord.co.jp>
Acked-by: Thor Thayer <thor.thayer@linux.intel.com>
[wsa: updated commit message]
Signed-off-by: Wolfram Sang <wsa@kernel.org>