input: synaptics: fix for buggy code poined by SIL tool

Place file offset validity checks under mutex for
synaptics_dsx_rmi_dev.c touch driver.

Git-repo: https://android.googlesource.com/kernel/msm


Git-commit: e1fb1600fc222337989e3084d68df929882deae5
Change-Id: I2c32babbccb483547204cb2843973abf97e988a5
Signed-off-by: Andrew Chant <achant@google.com>
[srkupp@codeaurora.org: This change is a fix for buggy
code pointed by sil after merging the above commit.}
Signed-off-by: Srinivasa Rao Kuppala <srkupp@codeaurora.org>
Signed-off-by: Shantanu Jain <shjain@codeaurora.org>