staging: sm7xxfb: fix error handling

We were checking smtc_regbaseaddress and that too at a place where it
can never be NULL. Real check should be on sfb->lfb immediately after
we do ioremap.

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