net: smc911x: Inline all functions from header file

Inline all the functions from the header file, as they are not used
outside of the driver or the standalone EEPROM example.

Note that this does introduce considerable amount of duplication in
the standalone EEPROM example, however that one has to be rewritten
anyway, roughly such that the SMC911x driver would expose DM EEPROM
interface and the standalone example would use that.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>