serial: 8250: extract serial8250_THRE_test()

serial: 8250: extract serial8250_THRE_test()

serial8250_do_startup() contains a stand-alone code for probing THRE.
Furthermore, the code block is conditional (port->irq and test for
UPF_NO_THRE_TEST).

Move this code to a separate function. The conditional can be evaluated
easier there -- by a simple return in the beginning. So the indentation
level lowers and the code is overall more readable now.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20250611100319.186924-16-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>