spi: xilinx_spi: Correct the fifo-depth calculation

spi: xilinx_spi: Correct the fifo-depth calculation

Correctly fill the fifo depth by passing correct node in getting
it from device tree. This fixes the issue of hanging in a loop while
reading environment from spi flash during boot up which is caused by
commit "spi: xilinx_spi: Modify transfer logic for quad mode"
(sha1: c638e0e80e5ddfac2999692f4aa824021bf3f196)

Signed-off-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>