staging: sm750fb: correctly define SM750LE_REVISION_ID

check if it is already defined before defining SM750LE_REVISION_ID
again and at the same time mention correct data type.

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>