usb: mdm_data_bridge: Add a module parameter for rmnet rx

Add a module parameter rx_rmnet_buffer_size so that
rmnet rx buffer size can be modified from user space.

Change-Id: I7ae148d85306f2d5a8fb55f5da64d4e7c52b0b04
Signed-off-by: Guang Jian Zeng <gzeng@codeaurora.org>