Bluetooth: shell: Add support for printing the write rate

This makes the gatt metrics also available for
gatt write-without-rsp-cb so it now prints the rate of each write:

uart:~$ gatt write-without-response-cb 1e ff 10 10
Write #1: 16 bytes (0 bps)
Write #2: 32 bytes (3445948416 bps)
Write #3: 48 bytes (2596929536 bps)
Write #4: 64 bytes (6400 bps)
Write #5: 80 bytes (8533 bps)
Write #6: 96 bytes (10666 bps)
Write #7: 112 bytes (8533 bps)
Write #8: 128 bytes (9955 bps)
Write #9: 144 bytes (11377 bps)
Write #10: 160 bytes (7680 bps)
Write #11: 176 bytes (8533 bps)
Write #12: 192 bytes (9386 bps)
Write Complete (err 0)
Write #13: 208 bytes (8533 bps)
Write #14: 224 bytes (9244 bps)
Write #15: 240 bytes (9955 bps)
Write #16: 256 bytes (8000 bps)

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>