xilinx_timer: Fix writes into TCSR register

The TCSR register has only 11 valid bits. This is now used by the
linux kernel to auto-detect endianness, and causes Linux 3.15-rc1
and later to hang when run under qemu-microblaze. Mask valid bits
before writing the register to solve the problem.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>

hw/timer/xilinx_timer.c

@@ -169,7 +169,7 @@ timer_write(void *opaque, hwaddr addr,
             if (value & TCSR_TINT)
                 value &= ~TCSR_TINT;
 
-            xt->regs[addr] = value;
+            xt->regs[addr] = value & 0x7ff;
             if (value & TCSR_ENT)
                 timer_enable(xt);
             break;