nrf/mphalport: Remove need for "syntax unified" in mp_hal_delay_us.

Because it can confuse older versions of gcc.  Instead use the correct
instruction for Thumb vs Thumb-2 (sub vs subs) so the assembler emits the
2-byte instruction.

Related to commit 1aa9ff914194824a78a8b010572ad7083c1bb4ec.

ports/nrf/mphalport.c

@@ -99,11 +99,12 @@ void mp_hal_delay_us(mp_uint_t us) {
     }
     register uint32_t delay __ASM("r0") = us;
     __ASM volatile (
-        #ifdef NRF51
-        ".syntax unified\n"
-        #endif
         "1:\n"
+        #ifdef NRF51
+        " SUB %0, %0, #1\n"
+        #else
         " SUBS %0, %0, #1\n"
+        #endif
         " NOP\n"
         " NOP\n"
         " NOP\n"