msm-geni-serial: Correct the unsupported baudrate error handling

Change the data-type of the variable used to hold the return value
of (get_clk_div_rate) api from unsigned int to int. This avoids the
implicit type-cast, which might result in improper error handling.

Change-Id: If780d9085acc33d27c2626936deeabd58aa5d7dd
Signed-off-by: Ashish Kori <akori@codeaurora.org>