drivers: gpio: renesas_ra_ioport: simplify pin configuration logic

- Removed zero initialization of `pincfg` structure as all members
  are guaranteed to be set.
- Introduced `pfs_cfg` as an intermediate variable to store data in
  the CPU register instead of stack.
- Simplified pin setting logic by relying on `pfs_cfg` being
  zero-initialized, eliminating the need for explicit bit clearing.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>