scsi: ufs-mediatek: Avoid using ret uninitialized in ufs_mtk_setup_clocks

scsi: ufs-mediatek: Avoid using ret uninitialized in ufs_mtk_setup_clocks

When building with -Wsometimes-uninitialized, Clang warns:

drivers/scsi/ufs/ufs-mediatek.c:112:7: warning: variable 'ret' is used
uninitialized whenever 'if' condition is false
[-Wsometimes-uninitialized]
                if (on)
                    ^~
drivers/scsi/ufs/ufs-mediatek.c:120:9: note: uninitialized use occurs
here
        return ret;
               ^~~
drivers/scsi/ufs/ufs-mediatek.c:112:3: note: remove the 'if' if its
condition is always true
                if (on)
                ^~~~~~~
drivers/scsi/ufs/ufs-mediatek.c:108:7: warning: variable 'ret' is used
uninitialized whenever 'if' condition is false
[-Wsometimes-uninitialized]
                if (!on)
                    ^~~
drivers/scsi/ufs/ufs-mediatek.c:120:9: note: uninitialized use occurs
here
        return ret;
               ^~~
drivers/scsi/ufs/ufs-mediatek.c:108:3: note: remove the 'if' if its
condition is always true
                if (!on)
                ^~~~~~~~
drivers/scsi/ufs/ufs-mediatek.c:96:9: note: initialize the variable
'ret' to silence this warning
        int ret;
               ^
                = 0
2 warnings generated.

Remove the default case and initialize ret to -EINVAL to properly fix
this warning.

Fixes: 6b4cae5d524e ("scsi: ufs-mediatek: Add UFS support for Mediatek SoC chips")
Link: https://github.com/ClangBuiltLinux/linux/issues/426
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Stanley Chu <stanley.chu@mediatek.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>