staging/fwserial: Fix compiler warning

Fix:
drivers/staging/fwserial/fwserial.c:581:3: warning: format '%ld' expects argument of type 'long int', but argument 4 has type 'size_t'

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>