spi: zynq-qspi: Dont write more than requested bytes to rxbuf

In case of dual parallel odd byte transfer, write even bytes and
read bytes accordingly but write only the requested number of bytes to rxbuf.
Otherwise any user data adjacent to rxbuf might be corrupted.
Improve related comments.

Signed-off-by: Harini Katakam <harinik@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>