brcmfmac: remove set but not used variable 'old_state'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/wireless/broadcom/brcm80211/brcmfmac/usb.c: In function 'brcmf_usb_state_change':
drivers/net/wireless/broadcom/brcm80211/brcmfmac/usb.c:578:6: warning:
 variable 'old_state' set but not used [-Wunused-but-set-variable]

It's never used and can be removed.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>