mtd: m25p80: consider max message size in m25p80_read

Consider a message size limit when calculating the maximum amount
of data that can be read.

The message size limit has been introduced with 4.9, so cc it
to stable.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>