clk: st: Fix error paths and allocation style

The error paths in this file leak memory and mappings and test
for pointers being valid after dereferencing them. Fix these
problems and properly free resources on errors. Fix some
stylistic things too like using sizeof(*ptr) and fitting more
code on a single line. Note that we don't unregister clocks here.
That needs a clk_composite_unregister() API that we don't have
right now.

Acked-by: Gabriel Fernandez <gabriel.fernandez@linaro.org>
Cc: Pankaj Dev <pankaj.dev@st.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>