omap: Ptr "isr_reg" tracked as NULL was dereferenced

Value of "isr_reg" pointer is depend on configuration and GPIO method.
Potentially it may have NULL value and it is dereferenced later
in code. Warning and exit from function are added in this case.

Signed-off-by: Evgeny Kuznetsov <EXT-Eugeny.Kuznetsov@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>