[MTD] fix CFI point method for discontiguous maps

The CFI probe routine is capable of detecting flash banks consisting of
identical chips mapped to physically discontiguous addresses.  (One
common way this can occur is if a flash bank is populated with chips of
less capacity than the hardware was designed to support.)  The CFI
point() routine currently ignores any such gaps.  This patch fixes
the CFI point() routine so that it truncates any request that would
span a gap.

Signed-off-by: Andy Lowe <alowe@mvista.com>
Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>