libsdhcdrivers: Change mmc_card_capacity type

mmc_card_capacity will now return signed value due to the following
reasons:

1. Function returns negative value in case of the failure, so naturally
   signed type must be used.
2. Signed 64 bit value is enough for holding the theoretical SD card
   size of 8 EiB (2^63 = 2^3 * 2^60 = 2^3 * 1024 ^ 6) what is more than
   enough when taking into account that the latest SDUC format maximum
   size is 128 TiB (2^7 * 1024^4).
2 files changed