spi: dw: Avoid BUG_ON() in case of host failure

If dws is NULL in dw_spi_host_add(), we return the error to the
upper callers instead of crashing. The patch replaces BUG_ON by
returning -EINVAL to the caller.

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Link: https://lore.kernel.org/r/20191205231421.9333-1-pakki001@umn.edu
Signed-off-by: Mark Brown <broonie@kernel.org>