ntb_hw_switchtec: NT req id mapping table register entry number should be 512

The number of available NT req id mapping table entries per NTB control
register is 512. The driver mistakenly limits the number to 256.

Fix the array size of NT req id mapping table.

Fixes: c082b04c ("NTB: switchtec: Add NTB hardware register definitions")
Signed-off-by: Wesley Sheng <wesley.sheng@microchip.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>