Revert "serial: ns16550: fix debug uart putc called before init"

This reverts commit 6f57c344 since it
does not seem to work at least on rk3399.

The Rockchip Technical Reference Manual (TRM) for the rk3399 says the baud
rate prescaler register is readable only when USR[0] is zero. Since this
bit is defined as "reserved" in the socfpga cylcone5 TRM, let's rather
drop this than making the ns16550 debug uart more platform specific.

Reported-by: Roosen Henri <Henri.Roosen@ginzinger.com>
Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>