drm/xe: Prefer USEC_PER_SEC over MICRO

It will be easier to understand the meaning of the flr_timeout
value if the USEC_PER_SEC macro is used in the expression.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250311140115.2042-1-michal.wajdeczko@intel.com