brcmfmac: of: Use devm_kstrdup for board_type & check for errors

This was missing a NULL check, and we can collapse the strlen/alloc/copy
into a devm_kstrdup().

Reviewed-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Hector Martin <marcan@marcan.st>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220131160713.245637-8-marcan@marcan.st