pinctrl: intel: refine ->set_mux() hook

pinctrl: intel: refine ->set_mux() hook

Utilize a temporary variable for common shift operation in
->set_mux() hook and improve readability while saving a few bytes.

add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-3 (-3)
Function                                     old     new   delta
intel_pinmux_set_mux                         245     242      -3
Total: Before=10472, After=10469, chg -0.03%

Signed-off-by: Raag Jadav <raag.jadav@intel.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Link: https://lore.kernel.org/r/20230613085054.10976-2-raag.jadav@intel.com
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>