Litex: clock: Undefined behavior due to unchecked return code

Found via static analysis. Two separate error paths where failing to
check the return code leads to undefined behavior:
1. In `litex_clk_get_phase`, the divider stays set to 0 when
   `litex_clk_get_clkout_divider` errors out, which leads to a division
by 0.
2. In `litex_clk_calc_duty_normal`, the `duty` struct is used
   uninitialized if `litex_clk_get_duty_cycle` errors out.

In both case, checking the return code and returning early resolves the
issue.

Signed-off-by: François Baldassari <francois@memfault.com>