flash_lock: fix length being passed

flash_lock: fix length being passed

This patch fixes the 'length' calculation.
Making it:
+       mtdLockInfo.length = (num_sectors - 1) * mtdInfo.erasesize;
Rather:
-       mtdLockInfo.length = num_sectors * mtdInfo.erasesize;

Say there are 240 blocks present in the device. Then:
offset starts from: 0x0
and full size of device: 0x1E00000

doing: 240 * 0x20000 gives -> 0x1E00000
But last block address should be 0x1DE0000 (which spans for 0x20000
bytes, adding upto size of 0x1E00000)

Signed-off-by: Vimal Singh <vimalsingh@ti.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>