Baud rate generation is inexact

When a baud rate is set in UART, SPI, or I2C, mbed-drivers does not provide feedback of the actual baud rate selected by hardware. Reporting of errors in this scenario requires one of three approaches:
- assert that baud rate is out of tolerance
- return an error code when baud rate is out of tolerance
- provide a query mechanism for baud rates.

cc @mjs-arm @niklas-arm @bogdanm @0xc0170

Start by examining the UART, SPI, and I2C baud-rate setting entry points and how each currently handles the hardware-selected rate. The issue lists three possible approaches—assertion, an error code, or a query mechanism—so a maintainer must first choose and define the expected behavior before implementation can be considered done.

Written by the indexing model from the issue text.