use xfer3 instead of xfer2

[muningis]

xref2 only allows hardcoded limit of 4096 for buffer size
xfer3 read bufsize from /sys/module/spidev/parameters/bufsiz instead of using hardcoded 4096 as mentioned above

References:

• Add xfer3 that supports arbitrary large lists doceme/py-spidev#75
• increased spi buffer sizer doceme/py-spidev#62

Alternatively, it could be dynamic, based on an option pased to constructor.

[vanshksingh]

Thank you !