Fix UARTx_S1 flag clearing in serial_lld.c.

The flags in S1 are write-1-to-clear, so the |= operation was actually
clearing *all* set flags in the S1 register regardless.
Merge pull request #3 from cdbennett/KL2x_serial_flag_clearing

Fix UARTx_S1 flag clearing in serial_lld.c.