[sw, dif_uart] Simplify the disable routine
[sw, dif_uart] Simplify the disable routine

This is a minor change, which doesn't change the behaviour of the
function.

There is no need for reading interrupt enable register when the passed
in state is NULL.

Signed-off-by: Silvestrs Timofejevs <5a6466a3751cccdb6ee413d1e87698ed8baaba81@lowrisc.org>

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