net: phy: nxp-c45-tja11xx: use bitmap_empty() where appropriate

The driver opencodes bitmap_empty() in a couple of funcitons. Switch to
the proper and more verbose API.

Signed-off-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>
Link: https://patch.msgid.link/20250913182837.206800-1-yury.norov@gmail.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>