mtd: spi-nor: xilinx: correct the debug message

XRDSR is a combination of xilinx and the RDSR opcode, but the register
is just the status register. Correct the debug message.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Pratyush Yadav <p.yadav@ti.com>
Link: https://lore.kernel.org/r/20220223134358.1914798-24-michael@walle.cc