mfd: Correct the name and bitmask for ab8500-gpadc BTempPullUp

The bitmask for enabling the BatTemp pull-up was wrong and
is corrected. The name is also changed to be inline with
the AB8500 register description

Signed-off-by: Johan Palsson <johan.palsson@stericsson.com>
Reviewed-by: Mattias Wallin <mattias.wallin@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>