arch/arm/rp23xx: Fix the nxstyle error in rp23xx_dmac.c.

nxstyle reports "Missing blank line after declarations" in
rp23xx_dmachannel().  Add the blank line, because CI checks every file
that a change touches.

No functional change.  The change is whitespace only.

Assisted-by: Claude Code:claude-opus-5-5
Signed-off-by: Marco Casaroli <marco.casaroli@gmail.com>

arch/arm/src/rp23xx/rp23xx_dmac.c

@@ -238,6 +238,7 @@ DMA_HANDLE rp23xx_dmachannel(void)
   for (ch = 0, dmach = NULL; ch < RP23XX_DMA_NCHANNELS; ch++)
     {
       struct dma_channel_s *candidate = &g_dmach[ch];
+
       if (!candidate->inuse)
         {
           dmach        = candidate;