i2c/designware: Fix an initialization issue

The i2c_dw_xfer_init() function requires msgs and msg_write_idx from the
dev context to be initialized.

amd_i2c_dw_xfer_quirk() inits msgs and msgs_num, but not msg_write_idx.

This could allow an out of bounds access (of msgs).

Initialize msg_write_idx before calling i2c_dw_xfer_init().

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Fixes: 17631e8c ("i2c: designware: Add driver support for AMD NAVI GPU")
Cc: <stable@vger.kernel.org> # v5.13+
Signed-off-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>
Link: https://lore.kernel.org/r/20250627143511.489570-1-michael.j.ruhl@intel.com