serial: mchp_xec: Fix uart_xec_irq_tx_complete function

[perezpri]

This function is only checking for byte current byte being transmitted. This patch ensures that function returns 1 until TX is disabled and no byte is being transmitted.

[perezpri]

CC: @scottwcpg

[scottwcpg]

It works. Note: old and new code could be simplified. TEMT bit[6] of LSR becomes 1 when both tx holding register AND tx shift register are empty. Depends on your purpose. Are you wanting to know if a new byte can be written to the UART TX reg or are you wanting to know if TX is finished transmitting and no longer requires a clock?

[perezpri]

It works. Note: old and new code could be simplified. TEMT bit[6] of LSR becomes 1 when both tx holding register AND tx shift register are empty. Depends on your purpose. Are you wanting to know if a new byte can be written to the UART TX reg or are you wanting to know if TX is finished transmitting and no longer requires a clock?

I want to know if TX has finished transmitting and no longer requires clock. In chrome EC, this function is used by uart_tx_flush to block current thread until all data is transmitted.

[perezpri]

CC: @keith-zephyr

[kartben]

Ping @jvasanth1