drm/i915/dp: Refactor max_vswing_tries variable

Changes the type and renames the max_vswing_tries variable
which was declared as an integer but used as a boolean
making it easy to be confused with a counter.

Changes in v2:
    - updated the title and commit message
    - left the loop exit point in place

v3: fix typo in title
v4: renamed max_vswing to max_vswing_reached (Ville)

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Marc Herbert <marc.herbert@intel.com>
Signed-off-by: Nathan Ciobanu <nathan.d.ciobanu@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180720214413.29506-2-rodrigo.vivi@intel.com