soc: ti: knav_dma: Remove unused DMA_PRIO_MASK macro

soc: ti: knav_dma: Remove unused DMA_PRIO_MASK macro

DMA_PRIO_MASK (GENMASK(3, 0)) is defined alongside the other priority
macros but is never referenced in the code. tx_priority and rx_priority
are only ever assigned DMA_PRIO_DEFAULT (0) and are never sourced from
device tree or user-controlled input, so no out-of-range value is
possible. W=2 builds report:

  knav_dma.c:32:9: warning: macro is not used [-Wunused-macros]
  32 | #define DMA_PRIO_MASK           GENMASK(3, 0)

Remove the dead macro.

Reviewed-by: Sai Sree Kartheek Adivi <s-adivi@ti.com>
Reviewed-by: Hari Prasath Gujulan Elango <gehariprasath@ti.com>
Link: https://patch.msgid.link/20260512170623.3174416-9-nm@ti.com
Signed-off-by: Nishanth Menon <nm@ti.com>