mtd: devices: Fix for qcs405 64bit compilation issue.

Chage the type of len variable from uint32_t to size_t to
resolve [-Werror,-Wincompatible-pointer-types] errors during
64bit compilation.

Change-Id: I5dfc6fb145fc96f9eef8ed9ec2e84ad3081ecb61
Signed-off-by: Pradeep P V K <ppvk@codeaurora.org>