net: dm9000: Drop dm9000.h and staticize SROM access

Dispose of dm9000.h because none of the function prototypes declared in
it are called anywhere in the codebase. Staticize dm9000_read_srom_word()
because it is now called only from within the dm9000 driver. Drop
dm9000_write_srom_word() because it is no longer used.

Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Ramon Fried <rfried.dev@gmail.com>