spi: mxc_spi: Fix double incrementing read pointer for unaligned buffers

If dout buffer is not 32 bit-aligned or data to transmit is not multiple
of 32 bit the read data pointer is already incremented on single byte reads.

Signed-off-by: Timo Herbrecher <t.herbrecher@gateware.de>
Signed-off-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>