[spinel] map kMaxSpinelFrame to SPINEL_FRAME_MAX_SIZE (#7580)

The current implementation does not differentiate the max spinel frame
size that is used to pack a spinel command and the radio spinel RX
frame buffer size which could contain multiple frames.  In
radio_spinel_impl.hpp, only one spinel frame can be written when
calling function `SendCommand` or `SendReset`.

Therefore the aim of this commit is to allocate, in such a context,
only a buffer of one frame instead of a buffer that could contain
multiple frames.

Signed-off-by: Gatien Chapon <[email protected]>

src/lib/spinel/radio_spinel.hpp

@@ -858,7 +858,7 @@ public:
 private:
     enum
     {
-        kMaxSpinelFrame        = SpinelInterface::kMaxFrameSize,
+        kMaxSpinelFrame        = SPINEL_FRAME_MAX_SIZE,
         kMaxWaitTime           = 2000, ///< Max time to wait for response in milliseconds.
         kVersionStringSize     = 128,  ///< Max size of version string.
         kCapsBufferSize        = 100,  ///< Max buffer size used to store `SPINEL_PROP_CAPS` value.