usb: core: update comments for send message functions

The commonly use of bottom halves are tasklet and workqueue. The big
difference between tasklet and workqueue is that the tasklet runs in
an interrupt context and the workqueue runs in a process context,
which means it can sleep if need be.

The comment for usb_control/interrupt/bulk_msg() functions note that do
not use this function within an interrupt context, like a 'bottom half'
handler. With this comment, it makes confuse about usage of these
functions.

To more clarify, remove 'bottom half' comment.

Signed-off-by: Jaejoong Kim <climbbb.kim@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>