libc: atoi() avoid fallthroughs in switch cases

This is a tentative fix for CID 93807, a valid Coverity warning;
however the code is valid too.

We can choose to silence the warning or rewrite the code in a way that
makes it more verbose but keeps Coverity happy.

Coverity complains about doing an implicit fallthrough in switch case
statement. I prefer patching the code to make it explicit, as the
compiler will optimize out, to avoid having to constantly filter out
the checker's warnings.

Coverity-CID: 93807
Change-Id: I7be334d48567bf52fc2b21de043310e0f73b72db
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>