iwlwifi: mvm: check iwl_nvm_init return value

iwl_nvm_init() return value wasn't checked in some
path, which resulted in the following panic (if
there was some issue with the nvm):

Unable to handle kernel NULL pointer dereference at virtual address 00000004
pgd = d0460000
[00000004] *pgd=00000000
Internal error: Oops: 5 [#1] PREEMPT SMP
Modules linked in: iwlmvm(+) iwlwifi mac80211 cfg80211 compat [last unloaded: compat]
PC is at iwl_mvm_mac_setup_register+0x12c/0x460 [iwlmvm]
LR is at 0x2710
pc : [<bf50dd4c>]    lr : [<00002710>]    psr: 20800013
sp : d00cfe18  ip : 0000081e  fp : d006b908
r10: d0711408  r9 : bf532e64  r8 : d006b5bc
r7 : d01af000  r6 : bf39cefc  r5 : d006ab00  r4 : d006b5a4
r3 : 00000001  r2 : 00000000  r1 : d006a120  r0 : d006b860

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>