mtd: nand: pxa3xx_nand: sync pxa3xx_nand_set_sdr_timing()

Since the pxa3xx_nand driver was added there has been a discrepancy in
pxa3xx_nand_set_sdr_timing() around the setting of tWP_min and tRP_min.
This brings us into line with the current Linux code.

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>