fastboot: getvar: fix partition-size return value

The size returned by 'getvar partition-size' should be in bytes, not in
blocks as fastboot uses that value to generate empty partition when
running format [1].

Note that the function was already returning the proper size in bytes
for NAND devices (see struct part_info details).

[1]
https://android.googlesource.com/platform/system/core/+/refs/heads/android10-release/fastboot/fastboot.cpp#1500



Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>