nimble/drivers: Fix loop condition in nrf51 driver

This condition was suggesting that loop can break while om is NULL but
this loop breaks only when copied enough bytes and om is expected to
have enough space.

nimble/drivers/nrf51/src/ble_phy.c

@@ -238,7 +238,7 @@ ble_phy_rxpdu_copy(uint8_t *dptr, struct os_mbuf *rxpdu)
     om = rxpdu;
     dst = om->om_data;
 
-    while (om) {
+    while (true) {
         /*
          * Always copy blocks of length aligned to word size, only last mbuf
          * will have remaining non-word size bytes appended.