usb: avoid redundant cast of kmalloc() return value in OTi-6858 driver

In drivers/usb/serial/oti6858.c::pl2303_buf_alloc() the return value
of kmalloc() is being cast to "struct pl2303_buf *", but that need
not be done here since kmalloc() returns "void *".

Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>