mach-snapdragon: capsule_update: Fix eMMC detection for non-UFS devices

mach-snapdragon: capsule_update: Fix eMMC detection for non-UFS devices

Currently (since 2026.01-rc) on all SDM630/660 based devices this is
printed, after observing long boot delay (several seconds) before
executing preboot commands:

 QCOM-FMP: Failed to find boot partition

find_target_partition() function incorrectly assumes that eMMC is always
at number 0. In general you can't rely on device numbering to determine if
particular block device is eMMC or SD-card, because it depends on how
aliases are defined in device tree "chosen" node. Some SoCs have MMC
numbers starting at 1, not 0; so mmc1 is eMMC, mmc2 is SD-card.

Make eMMC detection reliable by using IS_SD() macro from mmc.h header.
Using this method target boot partition can be found successfully.
With debug prints enabled, this is printed:

 QCOM-FMP: skipped SD-Card (devnum 2)
 QCOM-FMP: Capsule update target: boot (disk 1:60)
 QCOM-FMP: DFU string: 'mmc 0=u-boot.bin part 1 60'

Without debug prints nothing is printed, no error about failure to find
boot partition.

Fixes: fe80a5f80095 ("mach-snapdragon: CapsuleUpdate: support all boot methods")
Signed-off-by: Alexey Minnekhanov <alexeymin@minlexx.ru>
Reviewed-by: Casey Connolly <casey.connolly@linaro.org>
Link: https://patch.msgid.link/20251107232935.283843-1-alexeymin@minlexx.ru
Signed-off-by: Casey Connolly <casey.connolly@linaro.org>