mtd: nand: pxa3xx-nand: fix random command timeouts

When 2 commands are submitted in a row, and the second is very quick,
the completion of the second command might never come. This happens
especially if the second command is quick, such as a status read
after an erase

This patch is taken from Linux:
'commit 21fc0ef9652f'
("mtd: nand: pxa3xx-nand: fix random command timeouts")

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>