serial: sifive: Fix sifive_serial_console_setup() section

commit 9b8fef63 upstream.

This function is called indirectly from the platform driver probe
function. Even if the driver is built in, it may be probed after
free_initmem() due to deferral or unbinding/binding via sysfs.
Thus the function cannot be marked as __init.

Fixes: 45c054d0

 ("tty: serial: add driver for the SiFive UART")
Cc: stable <stable@kernel.org>
Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Link: https://lore.kernel.org/r/20230624060159.3401369-1-samuel.holland@sifive.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>