sysfs: Implement SMBus read/write byte data when I2C function is not available

[jseely]

Helps with #15

This PR is the first step towards SMBus support.
Currently if I2C functionality isn't available on the I2C bus the Ioctl call fails with unsupported.

This implements Read/Write Byte Data SMBus calls for transactions with (len(r)==1 && len(w)==1) and (len(r)==0 && len(w)==2) respectively.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 34 lines in your changes missing coverage. Please review.
✅ Project coverage is 27.5%. Comparing base (9cd28b7) to head (760e25b).

Files with missing lines	Patch %	Lines
sysfs/i2c.go	0.0%	33 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main     #72     +/-   ##
=======================================
+ Coverage   27.2%   27.5%   +0.3%     
=======================================
  Files         96      96             
  Lines      11834    9514   -2320     
=======================================
- Hits        3220    2613    -607     
+ Misses      8479    6765   -1714     
- Partials     135     136      +1     

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[jseely]

@maruel I'm currently working on getting some tests in place around this code. Are there any other changes you would want made to this PR?