mmc: fix 1-byte reallocs (when it should have been sizeof device_t)

Reported by KASAN:
panic: ASan: Invalid access, 8-byte write at 0xfffffe00f0992610, RedZonePartial(1)
panic() at panic+0xb5/frame 0xffffffff86a595b0
asan_store8_noabort() at asan_store8_noabort+0x376/frame 0xffffffff86a59670
mmc_go_discovery() at mmc_go_discovery+0x6c61/frame 0xffffffff86a5a790
mmc_delayed_attach() at mmc_delayed_attach+0x35/frame 0xffffffff86a5a7b0
[snip]

Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit 13c63ae08df43d4e662b73ea3e05137fff88e639)