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

Change-Id: I1cc2fb5495c321869c10c9d4c030507b67711a3e
CRs-fixed: 256706
Signed-off-by: Jeegar Lakhani <jlakhani@codeaurora.org>