drivers/clock/rcar: Move ASSERT to module clock function

ASSERT was failing when `r8a7795_cpg_mssr_start_stop` was
called for a "core" clock.

This ASSERT statement and "mstpcr" table of registers are
only meant to be used when starting or stopping a "module" clock.

Moved ASSERT statement to `rcar_cpg_mstp_clock_endisable`
as well as "reg" & "bit" calculation.

Signed-off-by: Aymeric Aillet <aymeric.aillet@iot.bzh>