qed: Avoid implicit enum conversion in qed_roce_mode_to_flavor

Clang warns when one enumerated type is implicitly converted to another.

drivers/net/ethernet/qlogic/qed/qed_roce.c:153:12: warning: implicit
conversion from enumeration type 'enum roce_mode' to different
enumeration type 'enum roce_flavor' [-Wenum-conversion]
                flavor = ROCE_V2_IPV6;
                       ~ ^~~~~~~~~~~~
drivers/net/ethernet/qlogic/qed/qed_roce.c:156:12: warning: implicit
conversion from enumeration type 'enum roce_mode' to different
enumeration type 'enum roce_flavor' [-Wenum-conversion]
                flavor = MAX_ROCE_MODE;
                       ~ ^~~~~~~~~~~~~
2 warnings generated.

Use the appropriate values from the expected type, roce_flavor:

ROCE_V2_IPV6 = RROCE_IPV6 = 2
MAX_ROCE_MODE = MAX_ROCE_FLAVOR = 3

While we're add it, ditch the local variable flavor, we can just return
the value directly from the switch statement.

Link: https://github.com/ClangBuiltLinux/linux/issues/125
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>