drm/xe/configfs: Drop MAX_GT_TYPE_CHARS constant

drm/xe/configfs: Drop MAX_GT_TYPE_CHARS constant

Early revisions of commit 7abd69278bb5 ("drm/xe/configfs: Add attribute
to disable GT types") used MAX_GT_TYPE_CHARS not only to size the
constant name field, but also for some of the string matching logic.  By
the time the patch finally landed, the constant was no longer needed for
parsing.  Stop using it for the string field definition as well; this
eliminates the risk that we forget to update the constant if we ever add
a GT type name longer than seven characters.

Suggested-by: Gustavo Sousa <gustavo.sousa@intel.com>
Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://patch.msgid.link/20251024200834.1512329-2-matthew.d.roper@intel.com
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>