drivers: ethernet: sam_gmac: Do not use "NTO" typedef style

The Atmel DFP headers define two "component typedef styles": RFO and
NTO; where the latter makes use of bit field structs to access hardware
registers.

The default component typedef style assumed by the DFP headers (i.e.
when `COMPONENT_TYPEDEF_STYLE` is not explicitly defined) is "RFO" and
this is indeed the component typedef style used throughout the Zephyr
Atmel SAM drivers, except in the particular instance which this commit
addresses.

The use of `GMAC_TA_Type` bit field struct, which is an "NTO" style
construct, is no longer possible with the latest DFPs because
conditional compilation checks for the bit field struct definitions
were added to restrict the use of such constructs to only when the
global component typedef style is set to "NTO".

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>