spi: cadence-quadspi: Fix cqspi_setup_flash()

The 'max_cs' stores the largest chip select number. It should only
be updated when the current 'cs' is greater than existing 'max_cs'. So,
fix the condition accordingly.

Also, return failure if there are no flash device declared.

Fixes: 0f3841a5 ("spi: cadence-qspi: report correct number of chip-select")
CC: stable@vger.kernel.org
Reviewed-by: Pratyush Yadav <pratyush@kernel.org>
Reviewed-by: Théo Lebrun <theo.lebrun@bootlin.com>
Signed-off-by: Santhosh Kumar K <s-k6@ti.com>
Message-ID: <20250905185958.3575037-4-s-k6@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>