mfd: ti-ssp: Fix build

    drivers/mfd/ti-ssp.c: In function 'ti_ssp_run':
    drivers/mfd/ti-ssp.c:286:2: error: implicit declaration of function 'set_current_state' [-Werror=implicit-function-declaration]
    drivers/mfd/ti-ssp.c:286:381: error: 'TASK_INTERRUPTIBLE' undeclared (first use in this function)
    drivers/mfd/ti-ssp.c:286:381: note: each undeclared identifier is reported only once for each function it appears in
    drivers/mfd/ti-ssp.c:286:2: error: implicit declaration of function 'signal_pending' [-Werror=implicit-function-declaration]
    drivers/mfd/ti-ssp.c:286:2: error: implicit declaration of function 'schedule' [-Werror=implicit-function-declaration]
    drivers/mfd/ti-ssp.c:286:2: error: implicit declaration of function '__set_current_state' [-Werror=implicit-function-declaration]
    drivers/mfd/ti-ssp.c:286:742: error: 'TASK_RUNNING' undeclared (first use in this function)
    drivers/mfd/ti-ssp.c: In function 'ti_ssp_interrupt':
    drivers/mfd/ti-ssp.c:311:32: error: 'TASK_NORMAL' undeclared (first use in this function)

Add missing #include <linux/sched.h>.

    drivers/mfd/ti-ssp.c: In function 'ti_ssp_probe':
    drivers/mfd/ti-ssp.c:412:12: error: 'struct mfd_cell' has no member named 'data_size'

data_size became unused in commit 40e03f57
("mfd: Drop data_size from mfd_cell struct"), hence remove its
initialization.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>