staging: tidspbridge: fix potential array out of bounds write

The name of the firmware (drv_datap->base_img) could potentially
become equal to 255 valid characters (size of exec_file), this
will result in an out of bounds write, given that the 255 chars
along with a '\0' terminator will be copied into an array of
255 chars.

Produce an error on this cases, because the driver expects the NULL
ending to be among the 255 char limit.

Reported-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: Omar Ramirez Luna <omar.ramirez@copitl.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>