serial: msm-geni-serial: Fix possible null pointer access with handle_rx

Some customers are using UART node in UFFI for console and using
the same node for HSUART in kernel but not as console. In this
scenario there is a possibility for null pointer access
with handle_rx while trying to stop secondary sequencer.
This change will move handle_rx initialization from port_startup to
probe function. This will help to avoid null pointer access issues.

Change-Id: Ibda592b375d14ba0c23c4b99223006b1fa53c211
Signed-off-by: Chandana Kishori Chiluveru <cchiluve@codeaurora.org>
Signed-off-by: Akash Asthana <akashast@codeaurora.org>