mtd-utils: Fix some simple cases of uninitialized value reads

This patch modifies the internal helpers to read and parse integers
from sysfs files by initializing them first and removes turns an
obscure "a = open(...) if (a >= 0) {...} if (a == -1) {...}" inside
recv_image into a more straight forward if/else branch.

Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>