stlink-common: Fix STM32L4 loader write count to reflect 64bits granularity

The stm32l4 loader expects a count of 32 bits words while its granularity is
really 64 bits.

This patch fixes this to simplify count calculation in run_flash_loader().

Signed-off-by: Maxime Coquelin <mcoquelin.stm32@gmail.com>