niash: increased the maximum Y extent to 297mm

297mm is advertised in the official specs for this machine family.
Tested against the ScanJet 3300C and it doesn't bottom out or anything.
Issue came about because A4 is 297mm in height and 296mm too short.
Probably just a typo or an oversight.

backend/niash.c

@@ -196,7 +196,7 @@ _UnityGammaTable (unsigned char *hwGamma)
 }
 
 static const SANE_Range rangeXmm = { 0, 220, 1 };
-static const SANE_Range rangeYmm = { 0, 296, 1 };
+static const SANE_Range rangeYmm = { 0, 297, 1 };
 static const SANE_Int startUpGamma = SANE_FIX (1.6);
 
 static const char colorStr[] = { SANE_VALUE_SCAN_MODE_COLOR };