Improved initialization and config upload

[joelsa]

This PR:

• fixes a TODO by making the prealloc buffer size configurable via const generics
• let's the driver take in a delay and uses that in the init function
• updates the Burst struct to take in values larger than 512
• udates the init function to be more careful and include more checks, heavily inspired by the steps from the official C driver

The reasons for this are:

• users might have much larger burst sizes
  • for example, I do my i2c development on a Linux machine with an i2c bus where I can do writes of 8192 bytes from userspace and with a custom driver or kernel module I could easily do a multiple of that
• I found the init function to not properly set up the internal ASIC of the BMI270. In particular, I found:
  • that the Softreset is mandatory before uploading the config
  • the delays are critical, especially the 20ms delay after finishing the config file upload and the 450us after dis- or enabling the advanced power save mode
  • writing of the config file needs to happen in two-bytes increments, there should be an odd number of bytes written in one go

[qrasmont]

Great! I'll test this ASAP

[qrasmont]

Here delay should be the 2nd argument not the last one and it should be Burst::new instead of Burst::Other. Otherwise LGTM!

[joelsa]

Good catch, will fix it!

[joelsa]

I've updated the Readme and the basic_read example in the latest commit.