serial/arc-uart: Fix section mistmatch

Commit "serial/arc-uart: split probe from probe_earlyprintk" introduced
a build time warning:

"WARNING: vmlinux.o(.data+0x5baa0): Section mismatch in reference from
the variable early_arc_platform_driver to the function
.init.text:arc_serial_probe_earlyprintk()"

While at it - fixed another incorrectly placed initdata annotation.

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: fengguang.wu@intel.com
Cc: linux-serial@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>