usb: u_data_bridge: Check allowable size with gbridge_port_write()

Currently gbridge_port_write() is not checking size of user space
provided data and directly copies buffer into USB request buffer
which is having size as 2KB. If user space application writes data
bigger than 2KB, copy_from_user() tries to copy data beyond allocated
USB request buffer size which results into memory corruption. Fix
this issue by checking allowable size with gbridge_port_write() and
making sure that it doesn't copy data beyond allocated buffer size.

Change-Id: I7a34e37f5d1609792374f07b70542b80edbe6944
Signed-off-by: Mayank Rana <mrana@codeaurora.org>