OPP: Fix potential null ptr dereference in dev_pm_opp_get_required_pstate()

In the Linux kernel, the following vulnerability has been resolved:

OPP: Fix potential null ptr dereference in dev_pm_opp_get_required_pstate()

"opp" pointer is dereferenced before the IS_ERR_OR_NULL() check. Fix it by removing the dereference to cache opp_table and dereference it directly where opp_table is used.

This fixes the following smatch warning:

drivers/opp/core.c:232 dev_pm_opp_get_required_pstate() warn: variable dereferenced before IS_ERR check 'opp' (see line 230)