Fix StatefulOutputPin

[jannic]

According to https://docs.rs/embedded-hal/latest/embedded_hal/digital/v2/trait.StatefulOutputPin.html,
the functions in StatefulOutputPin should not read the actual electrical
value of the pin, but the current state of the output driver.

The fe310 manual states: "Reading the output_val register returns the
written value. Reading the input_val register returns the actual value
of the pin gated by input_en."

So the right thing to do here is to read the corresponding bit in the
output register.

NOTE: I don't have hardware to actually test this fix.

[almindor]

NOTE: I don't have hardware to actually test this fix.

Sadly currently neither do I since I burned my redv :(

[yusefkarim]

I tested this on my RED-V, works fine 👍 .

An aside: I was wondering, whether if in the output functions (into_output, into_inverted_output, into_output_drive, and into_inverted_output_drive) we should also be clearing the input_en enable bit (e.g., adding the line $GPIOX::set_input_en(Self::INDEX, false);) in all the aforementioned functions?