drivers/pinctrl: ite: Don't clear FUNC3 setting unless alt_func is FUNC3

Previously, FUNC_3 related setting were cleared unconditionally,
regardless of the selected alternate function. This could
unintentionally disable FUNC_3 settings when configuring other
alternate functions.

This change ensures that FUNC_3 gcr/ext bits are only cleared
when alt_func is IT8XXX2_ALT_FUNC_3.

Signed-off-by: Tim Lin <tim2.lin@ite.corp-partner.google.com>