brcmfmac: simplify check finding NVRAM v1 device path

With a simple use of snprintf and small buffer we can compare NVRAM
entry value with a full string. This way we avoid checking random chars
at magic offsets.
Tested on BCM43602 with NVRAM hacked to use v1 format.

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