[POWERPC] iseries: Cleanup e2a() and strne2a()

e2a() was formally used by lparcfg, and so had to be exported, but isn't
anymore, so don't.

e2a() and strne2a() can both be static, and __init.

And e2a can be made much more concise if we use x ... y case labels, while
we're there add support for lower case letters.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>