SPI: cadence: Dont typecast master->num_chipselect

Use a local integer variable to get property and then write the same
in master->num_chipselect.

There is an option to use of_property_read_u16 but for
that different device-tree description has to be used
num-chip-select = /bits/ 16 <4>;
This change will be discussed in mainling process.

Signed-off-by: Harini Katakam <harinik@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>