samples: DTM: Add LE Uncoded PHYs into the conditions of VS commands

The VS commands are only supported on LE Uncoded PHYs.

For LE Coded PHY, when PKT field is set to 0b11,
it starts a transmitter test with packet payload 0xFF.

Signed-off-by: Ryan Chu <ryan.chu@nordicsemi.no>

Author: ryanjh

samples/bluetooth/direct_test_mode/README.rst

@@ -270,12 +270,13 @@ You can configure the antenna output and activation delay for the Skyworks front
 Vendor-specific packet payload
 ==============================
 
-The Bluetooth Low Energy 2-wire UART DTM interface standard reserves the Packet Type, also called payload parameter, with binary value ``11`` for a Vendor Specific packet payload.
+For the LE Uncoded PHYs, the Bluetooth Low Energy 2-wire UART DTM interface standard reserves the Packet Type, also called payload parameter, with binary value ``11`` for a Vendor Specific packet payload.
 
-The DTM command is interpreted as vendor-specific when both of the following conditions are met:
+The DTM command is interpreted as vendor-specific when the following conditions are met:
 
 * Its CMD field is set to Transmitter Test, binary ``10``.
 * Its PKT field is set to vendor-specific, binary ``11``.
+* The PHY is set to the LE Uncoded PHYs.
 
 Vendor-specific commands can be divided into different categories as follows: