fpga: machxo2-spi: Return an error on failure

Earlier successes leave 'ret' in a non error state, so these errors are
not reported. Set ret to -EINVAL before going to the error handler.

This addresses two issues reported by smatch:
drivers/fpga/machxo2-spi.c:229 machxo2_write_init()
  warn: missing error code 'ret'

drivers/fpga/machxo2-spi.c:316 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: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Moritz Fischer <mdf@kernel.org>