Fix hf sniff

This was broken in commit 17ab86c52 as the forced rounding up of
the size to 4-byte alignment in BigBuf_malloc made the size check
possibly larger than the buffer size as the check was always +3 on
the requested size rather than the rounded size. This was made
worse by BigBuf_max_traceLen not taking into account alignment
either and the alignmentn check in hfsnoop.c checking to 2 byte
alignment instead of 4 byte alignment.

The alignment size check now checks the size after alignment
rounding, and BigBuf_max_traceLen takes into account alignment
losses too reducing the need for BigBuf consumers to have to care
about alignment.

armsrc/hfsnoop.c

@@ -106,7 +106,7 @@ int HfSniff(uint32_t samplesToSkip, uint32_t triggersToSkip, uint16_t *len, uint
     FpgaWriteConfWord(FPGA_MAJOR_MODE_HF_SNIFF);
     SpinDelay(100);
 
-    *len = (BigBuf_max_traceLen() & 0xFFFE);
+    *len = BigBuf_max_traceLen();
     uint8_t *mem = BigBuf_malloc(*len);
 
     uint32_t trigger_cnt = 0;