env: mmc: Fix env loading with CONFIG_SYS_MMC_ENV_PART

When CONFIG_SYS_MMC_ENV_PART=2, the environment is loaded from
the USER partition (hwpart=0) instead of from the
BOOT1 partition (hwpart=2).

IS_ENABLED() cannot be used for non-boolean KConfig options.
Its documentation states:

> * IS_ENABLED(CONFIG_FOO) evaluates to 1 if CONFIG_FOO is set to 'y',
> * 0 otherwise.

So in our case, IS_ENABLED(CONFIG_SYS_MMC_ENV_PART) evaluates to 0.

Because of this, the hwpart variable is never assigned and mmc_offset()
ends up switching back to the USER hwpart (0) instead of BOOT1 (2).

Fix it by using #define instead.

Tested with:

  # have CONFIG_SYS_MMC_ENV_PART=2 in defconfig
  # 1. Erase mmc0boot1
  => mmc dev 0 2
  => mmc erase 0 400
  => mmc read ${loadaddr} 0 400
  => md ${loadaddr} 4
  82000000: 00000000 00000000 00000000 00000000  ................

  # 2. Restore default environment and save to MMC
  => env default -a -f
  => saveenv

  # 3. Read back mmc0boot1 and confirm the env is present
  => mmc read ${loadaddr} 0 400
  => md ${loadaddr} 4
  82000000: 13e0632e 72646461 7469665f 3978303d  .c..addr_fit=0x9

Fixes: 5b4acb0f ("env: mmc: Apply GPT only on eMMC user HW partition")
Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Reviewed-by: Tom Rini <trini@konsulko.com>