ubi-utils: Fix integer overflow in mtdinfo.c

ubi-utils: Fix integer overflow in mtdinfo.c

Report of the static analyzer:
The value of an arithmetic expression 'reginfo->offset + i * reginfo->erasesize' is a subject to overflow
because its operands are not cast to a larger data type before performing arithmetic

Corrections explained:
Added casting i and start to unsigned long long

Triggers found by static analyzer Svace.

Signed-off-by: Anton Moryakov <ant.v.moryakov@gmail.com>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>