iwlwifi: fix fw_pre_next_step to apply also for C step

C step NICs should use the latest FW (currently B step).
Correct the condition to make C step NICs advanced its default FW name
to the latest one.
Also rename _next_ to b_or_c to avoid confusion.

Fixes: 5da083d1

 ("iwlwifi: add support for 9000 HW B-step NICs")
Signed-off-by: Haim Dreyfuss <haim.dreyfuss@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>