OFFSET CAL register not working

[paul-thelu]

Hi there, First off thank you for the useful library. Just found an issue which prevent the user from using OFFSET CAL register.

Turns out the issue was a typo in mcp3564.h lines 232 to 234:

• "MCP3561_TIMER_ADDR" should be replaced by "MCP3561_OFFSETCAL_ADDR"

[mnemocron]

That was a good catch, thank you :)