usb: gadget: f_fs: Clean up ffs_log prints

Get rid of a number of ffs_log("exit") instances as they are not
adding much value for debugging and just take up space in the
log buffer. For a few other log sites add a bit more detail, such
as the ep number for I/O operations, or return value variable.

Also fix a couple of instances in which %zd (signed) sould have
been used instead of %zu (unsigned) for printing ssize_t values
to the log.

Change-Id: I5338aac7b3dba7b8ffae48afbb101f514a2e7573
Signed-off-by: Jack Pham <jackp@codeaurora.org>