Adapt to updated spidev kernel 3.15+ API
Adapt to updated spidev kernel 3.15+ API

New fields in spi_ioc_transfer struct: tx_nbits, rx_nbits
New IOCTLs: SPI_IOC_WR_MODE32, SPI_IOC_RD_MODE32 - we are checking for these to determine if the new fields are available in spi_ioc_transfer
see also https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit?id=dc64d39b54c1e9db97a6fb1ca52598c981728157

doceme/py-spidev,doceme/py-spidev