e1000e: mark shifted values as unsigned

The E1000_ICH_NVM_SIG_MASK value is shifted, out to the 31st bit, which
is the signed bit for signed constants. Mark these values as unsigned to
prevent compiler warnings and issues on platforms which a different
signed bit implementation.

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