ash: shell: Fix clang warnings about "string plus integer"

ash: shell: Fix clang warnings about "string plus integer"

Upstream commit:

    Date: Sat, 15 Dec 2018 18:49:31 +0100
    shell: Fix clang warnings about "string plus integer"

    Building with clang results in some warnings about integer values being
    added to strings.

    While the code itself is fine and the warnings are indeed harmless,
    fixing them also makes the semantic more explicit: what it is actually
    being increased is the address which points to the start of the string
    in order to skip the initial character when some conditions are met.

    Signed-off-by: Antonio Ospite <ao2@ao2.it>
    Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>