drivers/block/systemace - fixed data type in "systemace_read" to match prototype in "block_dev_desc_t"

Currently we have "unsigned long blkcnt" which is fine with
CONFIG_SYS_64BIT_LBA undefined because "lbaint_t" is basically the same
"unsigned long".
If CONFIG_SYS_64BIT_LBA gets defined "lbaint_t" is defined as "unsigned
long long".

Even though not many embedded systems have CONFIG_SYS_64BIT_LBA defined
it's good to have types in function implementation that match exactly
with prototypes.

Signed-off-by: Alexey Brodkin <alexey.brodkin@gmail.com>