staging: sm750fb: fix a type issue in sm750_set_chip_type()

"revId" needs to be unsigned because we use it to test:

	if (revId == SM750LE_REVISION_ID) {

and SM750LE_REVISION_ID is ((unsigned char )0xfe).

Fixes: 81dee67e

 ("staging: sm750fb: add sm750 to staging")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>