ufshci: initialize alloc_units before the dedicated-buffer scan

If every unit descriptor read failed in the LU-dedicated WriteBooster
scan, alloc_units was used uninitialized. Start it at zero so that case
is treated as a zero-sized buffer and WriteBooster is disabled.

Sponsored by:           Samsung Electronics
Reviewed by:		imp (mentor)
Differential Revision:	https://reviews.freebsd.org/D58663

sys/dev/ufshci/ufshci_dev.c

@@ -745,7 +745,7 @@ ufshci_dev_config_write_booster(struct ufshci_controller *ctrlr)
 {
 	struct ufshci_device *dev = &ctrlr->ufs_dev;
 	uint32_t extended_ufs_feature_support;
-	uint32_t alloc_units;
+	uint32_t alloc_units = 0;
 	struct ufshci_unit_descriptor unit_desc;
 	uint8_t lun;
 	bool is_life_time_left;