spi: imx: replace fixed timeout with calculated

Fixed timeout value can fire while transaction is ongoing. This may happen
because there are no strict requirements on SPI transaction duration.
Dynamic timeout value is generated based on SCLK and transaction size.

There is also 4 * SCLK delay between TX bursts related to HW internal CS change.

Signed-off-by: Anton Bondarenko <anton.bondarenko.sama@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>