iio: adc: stmpe-adc: Shuffle an if statement around in stmpe_adc_isr

When building with -Wsometimes-uninitialized, Clang warns:

drivers/iio/adc/stmpe-adc.c:204:13: warning: variable 'data' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]

Clang can't tell that data will never be used uninitialized because the
two if statements take care of all cases. Remove the first if statement
and make it the else branch of the second one so that it is apparent to
Clang that all cases are covered.

Link: https://github.com/ClangBuiltLinux/linux/issues/387


Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: NIck Desaulniers <ndesaulniers@google.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>