iwlwifi: mvm: add rtnl_lock() in iwl_mvm_start_get_nvm()

Due to a rebase damage, we lost the rtnl_lock() when the patch was
sent out.  This causes an RTNL imbalance and failed assertions, due to
missing RTNL protection, for instance:

  RTNL: assertion failed at net/wireless/reg.c (4025)
  WARNING: CPU: 60 PID: 1720 at net/wireless/reg.c:4025 regulatory_set_wiphy_regd_sync+0x7f/0x90 [cfg80211]
  Call Trace:
   iwl_mvm_init_mcc+0x170/0x190 [iwlmvm]
   iwl_op_mode_mvm_start+0x824/0xa60 [iwlmvm]
   iwl_opmode_register+0xd0/0x130 [iwlwifi]
   init_module+0x23/0x1000 [iwlmvm]

Fix this by adding the missing rtnl_lock() back to the code.

Fixes: eb09ae93

 ("iwlwifi: mvm: load regdomain at INIT stage")
Link: https://lore.kernel.org/lkml/CAHk-=wjB_zBwZ+WR9LOpvgjvaQn=cqryoKigod8QnZs=iYGEhA@mail.gmail.com/
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>