SPI: Properly aquire peripheral in constructor

[asmellby]

Prevent mismatch between _owner and peripheral configuration. In the previous
implementation, the following code would leave the peripheral in an inconsistent
state:

SPI spi1(...);      // _owner is NULL, peripheral config is 1
spi1.transfer(...); // _owner is 1, config is 1
SPI spi2(...);      // _owner is 1, config is 2
spi1.transfer(...)  // 1 thinks it still owns peripheral, doesn't reconfigure

@0xc0170

[0xc0170]

LGTM