usb: gadget: f_fs: Support multi-instance IPC logging

Currently the ffs_log() macro logs to a single IPC log buffer.
If multiple functions are instantiated (e.g. ADB and MTP) the
log would be extremely difficult to interpret. Instead, separate
each instance to its own IPC log buffer.

While at it, fix up the macro for ffs_log() to pass the exact
same string to ipc_log_string() and the printk/pr_debug(). This
is to ensure that the format string does not get duplicated with
in the compiled object.

Change-Id: Ieceea06c9b91ae8c2700e306d7ab5822e8a7a26c
Signed-off-by: Jack Pham <jackp@codeaurora.org>