bsps/shared/serial/pl011: Avoid unnecessary reads

Avoid reading from the pl011 data register unnecessarily. There is no
need to preserve the contents of this register as it is not normal
memory. This unnecessary read causes console spam when running under the
Xen hypervisor when the read FIFO is empty since the read is not
expected.

bsps/shared/dev/serial/arm-pl011.c

@@ -38,7 +38,7 @@ static inline void arm_pl011_write_char(
   const char ch
 )
 {
-  regs_base->uartdr = PL011_UARTDR_DATA_SET(regs_base->uartdr, ch);
+  regs_base->uartdr = PL011_UARTDR_DATA(ch);
 }
 
 static inline bool arm_pl011_is_rxfifo_empty(volatile pl011_base *regs_base)