arm64: percpu: Initialize the ret variable for default case

sometimes-uninitialized warning is observed, so define ret variable
for default case.

msm-4.14/arch/arm64/include/asm/percpu.h:102:1: warning: variable
'ret' is used uninitialized whenever switch default is taken
[-Wsometimes-uninitialized] error, forbidden warning: percpu.h:102

Change-Id: Ia364af84d3ba5862acdd62465d4f2219326b0c0b
Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.org>