twl4030-core: allow reading entire register banks

Minor change to the TWL4030 utility interface:  support reads
of all 256 bytes in each register bank (vs just 255).  This
can help when debugging, but is otherwise a NOP.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Tony Lindgren <tony@atomide.com>