msm_serial_hs: Fix the issue with device unregister/register

UART device can be removed/added dynamically based on the
requirement. Currently removing UART device using
platform_unregister_device(), device is crashing with NULL pointer
dereference error for debugfs entry node. Also add probe function
as __devinit instead of __init section.

Change-Id: I058c9093d79d90eb32fb34835cb4ebb1ef4fa704
Signed-off-by: Mayank Rana <mrana@codeaurora.org>