USART: Add methods to check rxne and txe bits

[ijager]

When USART is split into Rx/Tx objects.

Inspired by f4xx-hal

Usage:

if self.rx.is_rxne() {
    match self.rx.read() {
        Ok(byte) => {
            // do stuff
        },
        Err(_err) => {
             // handle uart read error
        }
    };
}

[andresv]

LGTM