SPI fix, new begin command and allow oneshotmode

[milesfrankland]

Fixes SPI issues some people were having by moving SPI.begin into a new mcp2515.begin() command. This also replaces mcp2515.reset() so you can remove 2 lines from the ino and replace with 1 easier one.
Added one shot mode sending enable by replacing a not used CANCTRL_REQOP_POWERUP (that was also incorrect in accordance with the spec sheet and not a mode - just a statment).

[manchoz]

Without this PR the library will fail at run-time on MbedOS-based Arduino boards (and actually works by chance on every other board)

Please, @autowp review and merge it!

[autowp]

Can't test for back compatibility. Are you checked that improvements on legacy boards?

[manchoz]

@autowp works as usual on legacy boards.

SPI (or I2C) initialization in the constructor has always been a cause of major issues in several Arduino libraries. This is the reason why almost every SPI-based library has a ::begin() method to be called before starting the actual operations.

As proposed by this PR, is a best practice for the constructor to just initialize the internal statuses and let the ::begin() and other ad-hoc methods to deal with external dependencies at run-time.

Thanks for your work and your time!

[autowp]

But now all library users must replace reset with begin. It' bc chage

[manchoz]

Uhm, I see.

Would be better to move theSPI.begin() to the ::reset() method, maybe? Would that be safer?