mcumgr: fix type of pointer passed to base64_decode

base64_decode requires 'size_t *olen', but '*int' was passed
instead. This caused a -Wincompatible-pointer-types "note: expected
'size_t *' {aka 'long unsigned int *'} but argument is of type 'int *'"
warning in qemu_x86 build. Fix that by using correct variable type.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>