hw/net/cadence_gem: Fix the mask/compare/disable-mask logic

Our current handling of the mask/compare logic in the Cadence
GEM ethernet device is wrong:
 (1) we load the same byte twice from rx_buf when
     creating the compare value
 (2) we ignore the DISABLE_MASK flag

The "Cadence IP for Gigabit Ethernet MAC Part Number: IP7014 IP Rev:
R1p12 - Doc Rev: 1.3 User Guide" states that if the DISABLE_MASK bit
in type2_compare_x_word_1 is set, the mask_value field in
type2_compare_x_word_0 is used as an additional 2 byte Compare Value.

Correct these bugs:
 * in the !disable_mask codepath, use lduw_le_p() so we
   correctly load a 16-bit value for comparison
 * in the disable_mask codepath, we load a full 4-byte value
   from rx_buf for the comparison, set the compare value to
   the whole of the cr0 register (i.e. the concatenation of
   the mask and compare fields), and set mask to 0xffffffff
   to force a 32-bit comparison

Signed-off-by: Andrew Yuan <andrew.yuan@jaguarmicro.com>
Message-id: 20241219061658.805-1-andrew.yuan@jaguarmicro.com
Suggested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
[PMM: Expand commit message and comment]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

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