spi: dw: Fix wrong FIFO level setting for long xfers

Due to using the u16 type in the min_t() macros the SPI transfer length
will be cast to word before participating in the conditional statement
implied by the macro. Thus if the transfer length is greater than 64KB the
Tx/Rx FIFO threshold level value will be determined by the leftover of the
truncated after the type-case length. In the worst case it will cause the
dramatical performance drop due to the "Tx FIFO Empty" or "Rx FIFO Full"
interrupts triggered on each xfer word sent/received to/from the bus.

The problem can be easily fixed by specifying the unsigned int type in the
min_t() macros thus preventing the possible data loss.

Fixes: ea11370f

 ("spi: dw: get TX level without an additional variable")
Reported-by: Sergey Nazarov <Sergey.Nazarov@baikalelectronics.ru>
Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20230113185942.2516-1-Sergey.Semin@baikalelectronics.ru
Signed-off-by: Mark Brown <broonie@kernel.org>