mfd: Fix error in wm8400 reg cache access check

Accessing num_reg elements in the interval [reg .. reg+num_regs)
is permitted if (reg+numregs <= array size), so barf when that
excluded upper bound is > array size. The prior -1 would give
access to one too many elements.

Signed-off-by: Phil Carmody <ext-phil.2.carmody@nokia.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>