nand/davinci: make sure ECC calculation has really started

Due to a register glitch (result code <4 might show up right after the
start-calculation-bit was set), make sure the ECC has really started.

See 1c3275b656045aff9a75bb2c9f3251af1043ebb3 in the kernel.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Cc: Sandeep Paulraj <s-paulraj@ti.com>