fm10k: cast page_addr to u8 * when incrementing it

The page_addr variable is a void pointer. Incrementing it before calling
prefetch is technically undefined. Fix this by casting it to a u8*
pointer before incrementing it. This ensures that we increment the
pointer value in byte units, instead of relying on this undefined
behavior.

This was detected by cppcheck, and resolves the following warning
produced by that tool:

[fm10k_main.c:328]: (portability) 'page_addr' is of type 'void *'. When
using void pointers in calculations, the behaviour is undefined.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>