mtd: nand: pxa3xx_nand: fix early spurious interrupt

When the nand is first probe, and upon the first command start, the
status bits should be cleared before the interrupts are unmasked.

This commit is taken from Linux:
'commit 0b14392db2e'
("mtd: nand: pxa3xx_nand: fix early spurious interrupt")

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Ofer Heifetz <oferh@marvell.com>
Reviewed-by: Igal Liberman <igall@marvell.com>
Cc: Stefan Roese <sr@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Stefan Roese <sr@denx.de>