shell: Fix shell uart not waking up shell on mcumgr data

When mcumgr smp data was received over shell uart transport
it was not waking up shell thread and thus request was not
processed. Shell thread must be waken up on any incoming
data, even data which is only dedicated for mcumgr smp.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>