Cleanup SPI constructor and add destructor

_acquire() is not required in constructor, since we are not performing
any operation on SPI bus yet. Just initialize the pins/hw

Destructor is required to clear _owner else SPI format/frequency will not be
set if object is recreated. We do not free SPI bus, but init again in hardware
may or may not change frequency/format.

```
{
    SPI spi1(...);
    spi1.transfer(...);
}
{
    SPI spi1(...);
    spi1.transfer(...);
}
```

drivers/SPI.cpp

@@ -41,9 +41,14 @@ SPI::SPI(PinName mosi, PinName miso, PinName sclk, PinName ssel) :
     _write_fill(SPI_FILL_CHAR)
 {
     // No lock needed in the constructor
-
     spi_init(&_spi, mosi, miso, sclk, ssel);
-    _acquire();
+}
+
+SPI::~SPI()
+{
+    if (_owner == this) {
+        _owner = NULL;
+    }
 }
 
 void SPI::format(int bits, int mode)

drivers/SPI.h

@@ -87,6 +87,7 @@ public:
      *  @param ssel SPI chip select pin
      */
     SPI(PinName mosi, PinName miso, PinName sclk, PinName ssel = NC);
+    virtual ~SPI();
 
     /** Configure the data transmission format
      *
@@ -272,11 +273,6 @@ private:
 
 #endif
 
-public:
-    virtual ~SPI()
-    {
-    }
-
 protected:
     spi_t _spi;