brcmsmac: fix u16 overflow warning

brcmsmac: fix u16 overflow warning

DOT11_MIN_BEACON_PERIOD and DOT11_MAX_BEACON_PERIOD are
superfluous. The only invalid beacon period is 0. Comparing
a 16 bit quantity to 0xffff also causes a compile warning:

drivers/net/wireless/brcm80211/brcmsmac/main.c:5560 brcms_c_set_beacon_period()
 warn: impossible condition '(period > 65535) => (0-65535 > 65535)'

Observed from smatch analysis.

Cc: Brett Rudley <brudley@broadcom.com>
Cc: Arend van Spriel <arend@broadcom.com>
Cc: "Franky (Zhenhui) Lin" <frankyl@broadcom.com>
Cc: Hante Meuleman <meuleman@broadcom.com>
Cc: "John W. Linville" <linville@tuxdriver.com>
Cc: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Cc: Seth Forshee <seth.forshee@canonical.com>
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Cc: linux-wireless@vger.kernel.org
Cc: brcm80211-dev-list@broadcom.com
Cc: netdev@vger.kernel.org
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Acked-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>