mtd: set read ahead value of read queue to 4KB

The read ahead value was set to 4 KB in the .29/.32 kernel since it was
found to be the optimal value for boot time. This setting isn't present
in the .35 kernel hence setting the read ahead value to 4 KB in the .35
kernel

CRs-fixed: 256706
Signed-off-by: Jeegar Lakhani <jlakhani@codeaurora.org>

Change-Id: Ia67bef6f07bd6889cab30b6a2150e8ab103c4b25