Convert CONFIG_NET_RETRY_COUNT to Kconfig

This converts the following to Kconfig:
   CONFIG_NET_RETRY_COUNT

Cc: Ramon Fried <rfried.dev@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/usb_a9263_dataflash_defconfig b/configs/usb_a9263_dataflash_defconfig
index 2dccfb1..1f34f33 100644
--- a/configs/usb_a9263_dataflash_defconfig
+++ b/configs/usb_a9263_dataflash_defconfig
@@ -37,6 +37,7 @@
 CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_ENV_SPI_MAX_HZ=15000000
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_NET_RETRY_COUNT=20
 CONFIG_DM=y
 CONFIG_CLK=y
 CONFIG_CLK_AT91=y