mailbox: Stop using ENOSYS for anything other than unimplemented syscalls

In accordance with e15f431f ("errno.h: Improve ENOSYS's comment") and
91c9afaf ("checkpatch.pl: new instances of ENOSYS are errors") we're
converting from the old meaning of: ENOSYS "Function not implemented" to
a more standard EINVAL.

Reported-by: Seraphin Bonnaffe <seraphin.bonnaffe@st.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>