Fix delay_ms in readme and const at reg_data for Write method

[cr0wg4n]

I was trying this driver in my esp8266 microcontroller and I've detect these errors, the readme documentation shows delay_ms is in the bme280_dev struct but is not, instead of delay_ms is delay_us.

At the compilation time this error arrives me: error: invalid conversion from 'int8_t (*)(uint8_t, uint8_t*, uint32_t, void*) I've review the code and I've detect the const type of variable is in the reg_data pointer at bme280_write_fptr_t typedef for write method.

I hope this PR help us, thank you so much.