phy: intel: fix enum type mismatch warning

clang points out that a local variable is initialized with
an enum value of the wrong type:

drivers/phy/intel/phy-intel-combo.c:202:34: error: implicit conversion from enumeration type 'enum intel_phy_mode' to different enumeration type 'enum intel_combo_mode' [-Werror,-Wenum-conversion]
        enum intel_combo_mode cb_mode = PHY_PCIE_MODE;
                              ~~~~~~~   ^~~~~~~~~~~~~

>From reading the code, it seems that this was not only the
wrong type, but not even supposed to be a code path that can
happen in practice.

Change the code to have no default phy mode but instead return an
error for invalid input.

Fixes: ac0a95a3 ("phy: intel: Add driver support for ComboPhy")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Dilip Kota <eswara.kota@linux.intel.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Link: https://lore.kernel.org/r/20200527134518.908624-1-arnd@arndb.de


Signed-off-by: Vinod Koul <vkoul@kernel.org>