serial: 8250_omap: extract omap_8250_set_termios_atomic()

serial: 8250_omap: extract omap_8250_set_termios_atomic()

To use guard()s easily in omap_8250_set_termios(), split it into atomic
and non-atomic part. The former can be easily guarded -- without a need
of indenting or moving code.

omap_8250_set_termios() would likely profit from a cleanup similar to
one in serial8250_do_set_termios() in commit cdc4a3e0b235 ("serial:
8250: extract serial8250_set_fcr()") and earlier.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20250814072456.182853-12-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>