i2c: Fix default RTIO handler transactions

Transactions from RTIO should result in single calls to i2c_transfer.
This corrects the default handler to first count the number of
submissions in the transaction, allocate on the stack, and then copy
over each submission to an equivalent i2c_msg.

It also cleans up the helper functions to be infallible, taking only the
submission and msg to copy to.

Signed-off-by: Tom Burdick <thomas.burdick@intel.com>