drivers: ipa: fix typos for ipa_smp2p structure doc

Remove the duplicate "mutex", and change "Motex" to "Mutex". Also I
recommend it's easier for understanding to make the "ready-interrupt"
a bundle for it is a parallel description as "shutdown" which is appended
after the slash.

Signed-off-by: Wang Wenhu <wenhu.wang@vivo.com>
Cc: Alex Elder <elder@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>