spi: cadence_qspi: Fix checking return value of fdt_first_subnode()

fdt_first_subnode() returns -FDT_ERR_NOTFOUND if no subnode found.
0 is supposed to be a valid offset returns from fdt_first_subnode().

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>