usb: gadget: remove useless cast

Remove the cast done by ERR_PTR() and PTR_ERR() since data is of type char
* and fss_prepare_buffer() should returns a value of this type.

Acked-by: Felipe Balbi <balbi@kernel.org>
Signed-off-by: Salah Triki <salah.triki@gmail.com>
Link: https://lore.kernel.org/r/20210731171838.GA912463@pc


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>