usb: isp1760: Use kasprintf

Use kasprintf instead of combination of kmalloc and sprintf and
therefore avoid unnecessary computation of string length.
Also, remove the useless local variable.

Signed-off-by: Himanshu Jha <himanshujha199640@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>