brcmfmac: simplify check stripping v2 NVRAM

Comparing NVRAM entry with a full filtering string is simpler than
comparing it with a short prefix and then checking random chars at magic
offsets. The cost of snprintf relatively low, we execute it just once.
Tested on BCM43602 with NVRAM hacked to use V2 format.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>