sdhci: Fix crash caused by M_WAITOK in sdhci dumps

In some contexts it is illegal to wait for memory allocation, causing
kernel panic. By default sbuf_new passes M_WAITOK to malloc,
which caused crashes when sdhci_dumpcaps or sdhci_dumpregs was callend in
non sutiable context.

Add SBUF_NOWAIT flag to sbuf_new to fix this.

Obtained from: Semihalf
Differential revision: https://reviews.freebsd.org/D32075