mfd: ti_am335x_tscadc: Fix static checker warning

It is possible that platform_get_resource() might return NULL and
therefore code needs to check for this condition before
de-referencing the pointer.

Therefore move the de-referencing of 'res' pointer after
devm_ioremap_resource() which would have checked the validity of the
pointer.

Reported-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>