usb: gadget: ffs: remove ENTER() macro

ENTER() used to show function name which called during runtime, ftrace can
be used to get same information, let's remove it.

Signed-off-by: Linyu Yuan <quic_linyyuan@quicinc.com>
Link: https://lore.kernel.org/r/1679911940-4727-1-git-send-email-quic_linyyuan@quicinc.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>