spi: zynqmp_gqspi: Fix unaligned data writes issue

When unaligned 3 bytes data write operation is performed, 3rd byte
is being over written by 1st byte of 3 bytes data. This patch
fixes it.

Signed-off-by: T Karthik Reddy <t.karthik.reddy@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>