staging: brcm80211: use int_sqrt kernel function iso driver implementation

The driver code had its own implementation for fixed point square root
calculation, but such a function is already available in the linux
kernel so the driver implementation has been removed using the kernel
function.

Cc: devel@linuxdriverproject.org
Cc: linux-wireless@vger.kernel.org
Reviewed-by: Roland Vossen <rvossen@broadcoom.com>
Reviewed-by: Henry Ptasinski <henryp@broadcom.com>
Reviewed-by: Brett Rudley <brudley@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>