fpga: machxo2-spi: Fix missing error code in machxo2_write_complete()

The error code is missing in this code scenario, add the error code
'-EINVAL' to the return value 'ret'.

Eliminate the follow smatch warning:

drivers/fpga/machxo2-spi.c:341 machxo2_write_complete()
  warn: missing error code 'ret'.

[mdf@kernel.org: Reworded commit message]
Fixes: 88fb3a00 ("fpga: lattice machxo2: Add Lattice MachXO2 support")
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Moritz Fischer <mdf@kernel.org>