pxa2xx: fix SSSR TFN logic

Fix SSSR TFN logic: TX FIFO is never filled, so it is always in
underrun condition if SSP is enabled.

This also avoids a gcc warning with -Wtype-limits.
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

hw/pxa2xx.c

@@ -636,6 +636,7 @@ static void pxa2xx_ssp_fifo_update(PXA2xxSSPState *s)
 {
     s->sssr &= ~(0xf << 12);	/* Clear RFL */
     s->sssr &= ~(0xf << 8);	/* Clear TFL */
+    s->sssr &= ~SSSR_TFS;
     s->sssr &= ~SSSR_TNF;
     if (s->enable) {
         s->sssr |= ((s->rx_level - 1) & 0xf) << 12;
@@ -643,14 +644,13 @@ static void pxa2xx_ssp_fifo_update(PXA2xxSSPState *s)
             s->sssr |= SSSR_RFS;
         else
             s->sssr &= ~SSSR_RFS;
-        if (0 <= SSCR1_TFT(s->sscr[1]))
-            s->sssr |= SSSR_TFS;
-        else
-            s->sssr &= ~SSSR_TFS;
         if (s->rx_level)
             s->sssr |= SSSR_RNE;
         else
             s->sssr &= ~SSSR_RNE;
+        /* TX FIFO is never filled, so it is always in underrun
+           condition if SSP is enabled */
+        s->sssr |= SSSR_TFS;
         s->sssr |= SSSR_TNF;
     }