brcmfmac: remove 'always false' condition from brcmf_c_mkiovar_bsscfg

The parameter buflen is unsigned so the condition buflen < 0 is
always false. The patch fixes the if statement checking the buffer
length.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>