serial: bfin_sport_uart: work around anomaly 05000473 (make 32bit fifo read atomic)

We cannot let a 32-bit RX FIFO read be interrupted otherwise a fake RX
underflow error might be generated.

URL: http://blackfin.uclinux.org/gf/tracker/5145



Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>