spi: cadence_qspi: Fix fdt read of spi-max-frequency

spi: cadence_qspi: Fix fdt read of spi-max-frequency

Fix the fdt read for spi-max-frequency as it's contained
in the child node. Current state of code is always
returning default value.

Signed-off-by: Chin Liang See <[email protected]>
Cc: Dinh Nguyen <[email protected]>
Cc: Dinh Nguyen <[email protected]>
Cc: Marek Vasut <[email protected]>
Cc: Stefan Roese <[email protected]>
Cc: Vikas Manocha <[email protected]>
Cc: Jagannadh Teki <[email protected]>
Cc: Pavel Machek <[email protected]>
Acked-by: Marek Vasut <[email protected]>
Acked-by: Pavel Machek <[email protected]>