usb: gadget: Validate the port number in grmnet_ctrl_smd_port_alloc

Fix code quality issues found by the automatic checking tool.
Verify that port number is within the max port limit before setting it
in grmnet_ctrl_smd_port_alloc.

CRs-Fixed: 566927
Change-Id: Iecc50a98c4e32bd8800ea63de0ccb8aecda78090
Signed-off-by: Danny Segal <dsegal@codeaurora.org>
Signed-off-by: Maya Erez <merez@codeaurora.org>