[iwm] Sync iwm_nvm_read_chunk() function with Linux iwlwifi.

This fixes an error handling detail in iwm_nvm_read_chunk(), where an
error response from the firmware for an NVM read shouldn't be fatal if
the offset was non-zero.

Approved by: adrian (mentor)
Obtained from: DragonFlyBSD git 250a1c33fca1725121fe499f9cebc90267d209f9
Differential Revision: https://reviews.freebsd.org/D8542