tty: serial: qcom_geni_serial: Cleanup redundant code

* Remove redundant casting while using min_t
* Remove redundant initialization of port_setup flag
* Remove redundant error checking in get_tx_fifo_size
* Remove logging redundant error code in debug messages
* Remove redundant disable_irq before free_irq

Signed-off-by: Karthikeyan Ramasubramanian <kramasub@codeaurora.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>