soc: ti: pm33xx: Remove set but unused variable 'ret'

Fixes the following W=1 kernel build warning(s):

 drivers/soc/ti/pm33xx.c: In function ‘am33xx_do_sram_idle’:
 drivers/soc/ti/pm33xx.c:138:6: warning: variable ‘ret’ set but not used [-Wunused-but-set-variable]

Cc: Santosh Shilimkar <ssantosh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>