msm: pcie: add bounds check for debugfs register write

Via debugfs nodes, users have the option to read and write to
any PCIe register. To ensure clients do not access registers
outside the PCIe range, add checks to validate the offset clients
provide.

Change-Id: Ia35cd04c57f01c21a47962be596bca395b5ca247
Signed-off-by: Tony Truong <truong@codeaurora.org>