[MFD] Fix gcc4 build errors in ucb1x00-core.c

drivers/mfd/ucb1x00-core.c:555: error: static declaration of 'ucb1x00_class' follows non-static declaration
drivers/mfd/ucb1x00.h:109: error: previous declaration of 'ucb1x00_class' was here

Since ucb1x00_class isn't used by anything, remove the extern
declaration and the symbol export.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>