net: stmmac: fix return value of __setup handler

__setup() handlers should return 1 on success, i.e., the parameter
has been handled. A return of 0 causes the "option=value" string to be
added to init's environment strings, polluting it.

Fixes: 47dd7a54 ("net: add support for STMicroelectronics Ethernet controllers.")
Fixes: f3240e28 ("stmmac: remove warning when compile as built-in (V2)")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: Igor Zhbanov <i.zhbanov@omprussia.ru>
Link: lore.kernel.org/r/64644a2f-4a20-bab3-1e15-3b2cdd0defe3@omprussia.ru
Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: Alexandre Torgue <alexandre.torgue@foss.st.com>
Cc: Jose Abreu <joabreu@synopsys.com>
Link: https://lore.kernel.org/r/20220224033536.25056-1-rdunlap@infradead.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>