brcmfmac: fix static checker warning in pmklist handling

The patch fixes a static checker warning:

   drivers/net/wireless/brcm80211/brcmfmac/cfg80211.c:2965
      brcmf_cfg80211_set_pmksa()
	warn: can 'pmkid_len' be negative?

The answer to the question above is likely no so changing its
type to unsigned is sufficient.

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