[sw, dif_uart] Remove null check for base_addr.base

The intention of the memio_region_t is to be an opaque
container. Having this check in place makes the build fail with
c_args: ['-DMOCK_MMIO'].

Signed-off-by: Silvestrs Timofejevs <silvestrst@lowrisc.org>
diff --git a/sw/device/lib/dif/dif_uart.c b/sw/device/lib/dif/dif_uart.c
index 51173f5..5308b48 100644
--- a/sw/device/lib/dif/dif_uart.c
+++ b/sw/device/lib/dif/dif_uart.c
@@ -169,7 +169,7 @@
 
 bool dif_uart_init(mmio_region_t base_addr, const dif_uart_config_t *config,
                    dif_uart_t *uart) {
-  if (uart == NULL || config == NULL || base_addr.base == NULL) {
+  if (uart == NULL || config == NULL) {
     return false;
   }