hw/smc91c111: Add explicit 'return' rather than relying on fallthrough

hw/smc91c111: Add explicit 'return' rather than relying on fallthrough

Add an explicit 'return' statement to a case in smc91c111_readb
rather than relying on fallthrough to the following case's
return statement, for code clarity and to placate static analysers.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>