net: stmmac: Remove redundant checking for rx_coalesce_usecs

net: stmmac: Remove redundant checking for rx_coalesce_usecs

The datatype of rx_coalesce_usecs is u32, always larger or equal to zero.
Previous checking does not include value 0, this patch removes the
checking to handle the value 0. This change in behaviour making the
value of 0 cause an error is not a problem because 0 is out of
range of rx_coalesce_usecs.

Signed-off-by: Gan Yi Fang <yi.fang.gan@intel.com>
Link: https://lore.kernel.org/r/20231018030802.741923-1-yi.fang.gan@intel.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>