omap2_mcspi: fix PIO RX_ONLY transfer

omap2_mcspi: fix PIO RX_ONLY transfer

Before transmission of the last word in PIO RX_ONLY mode rx+tx mode
is enabled:

	/* prevent last RX_ONLY read from triggering
	 * more word i/o: switch to rx+tx
	 */
	if (c == 0 && tx == NULL)
		mcspi_write_cs_reg(spi,
				OMAP2_MCSPI_CHCONF0, l);

But because c is decremented after the test, c will never be zero and
rx+tx will not be enabled. This breaks RX_ONLY mode PIO transfers.

Fix it by decrementing c already in the beginning of the loop.

Signed-off-by: Kalle Valo <kalle.valo@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>