iwlwifi: mvm: avoid harmless -Wmaybe-uninialized warning

gcc is apparently unablel to track the state of the local 'resp_v2'
variable across the kzalloc() function, and warns about the response
variable being used without an initialization:

drivers/net/wireless/intel/iwlwifi/mvm/nvm.c: In function ‘iwl_mvm_update_mcc’:
drivers/net/wireless/intel/iwlwifi/mvm/nvm.c:727:36: warning: ‘mcc_resp_v1’ may be used uninitialized in this function [-Wmaybe-uninitialized]
   resp_cp->n_channels = mcc_resp_v1->n_channels;
drivers/net/wireless/intel/iwlwifi/mvm/nvm.c:721:3: warning: ‘mcc_resp’ may be used uninitialized in this function [-Wmaybe-uninitialized]
   memcpy(resp_cp, mcc_resp, resp_len);

The warning showed up in x86 allmodconfig after my patch to
unhide -Wmaybe-uninitialized warnings by default was merged,
though it always existed in randconfig builds. I did not
catch the warning earlier because I was testing on ARM, which
never produced the warning.

This rearranges the code in a way that improves readability for
both humans and the compiler, and that avoids the warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 6fa52430 ("iwlwifi: mvm: change mcc update API")
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>