Staging: fbtft: Fix bug in fbtft-core

Commit 367e8560 introduced a bug
in fbtft-core where fps is always 0, this is because variable
update_time is not assigned correctly.

Signed-off-by: Ksenija Stanojevic <ksenija.stanojevic@gmail.com>
Fixes: 367e8560 ("Staging: fbtbt: Replace timespec with ktime_t")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>