mfd: twl-core: Fix incorrect function name twl_regcache_bypass()

Fixes the following W=1 kernel build warning(s):

 drivers/mfd/twl-core.c:496: warning: expecting prototype for twl_regcache_bypass(). Prototype was for twl_set_regcache_bypass() instead

Cc: Tony Lindgren <tony@atomide.com>
Cc: Lee Jones <lee.jones@linaro.org>
Cc: Kai Svahn <kai.svahn@nokia.com>
Cc: syed khasim <x0khasim@ti.com>
Cc: linux-omap@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>