staging: gs_fpgaboot: change char to u8

The bitstream storage variables were changed from char to u8 arrays to
prevent issues such as negative lengths. This change makes the code
compatible with the "data" field in "struct firmware" which is of type
u8.

Signed-off-by: Jacob von Chorus <jacobvonchorus@cwphoto.ca>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>